i386: stand: Tiny clean up for `start_pxeboot.o`, NFC

i386: stand: Retire its own nfs.c, and switch to libsa/nfs.c. NFC

branches: 1.28.26;
Incrase pxeboot code size limit from 64k to 128k. Fixes PR kern/54560.
The start_pxe.S part was suggested by mlelstv.

Consistently use ${RELEASEDIR}/${RELEASEMACHINE} instead of
${RELEASEDIR}/${MACHINE} (Paul Ripke)

Add multiboot 2 support to x86 bootloaders

multiboot 2 is required to boot Xen on an EFI system.
This also require a kernel patch for properly discovering
the ACPI RSDP, which is available after 20190912, in
src/sys/arch/x86/acpi/acpi_machdep.c 1.26-1.28

There are a few missing bit in this multiboot 2 implementation
(which are unused by Xen):
- Header tags Address, Freambuffer, and Relocatable are ignored
- Tags APM and Network are not provided
- Tags ACPI old and ACP new are only provided for ACPI boot
- Tag boot device does not provides the subpart (BSD disklabel partition)

Notes:
- multiboot2 is disabled in dosboot, otherwise the binary
gets too big and build fails.
- in src/sys/arch/i386/stand/efiboot, consinit() is renamed
as efi_consinit() to avoid prototype conflicts in src/sys/sys/systm.h

branches: 1.25.2; 1.25.6;
- Disable MKSANITIZER
- Redo using NOPIE
- Add NORELRO

branches: 1.24.10; 1.24.16;
We'll define the kernel types for standalone code.

Define _KERNTYPES for things that need it.

branches: 1.22.6;
Reduce amount of -no-integrated-as on x86 as .code16 is now supported by
LLVM.

branches: 1.21.4; 1.21.14; 1.21.18;
Disable integrated assembler for files that use .code16 or .code32 for
now. Disable Clang completely for bootxx and netboot for now until the
size issue has been sorted out.

branches: 1.20.4; 1.20.6;
fix tyop in default flag definition

Invert the flag bits to control module loading: rename
LOADMODULES->NOMODULES and READBOOTCONF->NOBOOTCONF.
This way, the default value (0) wired into old bootxx_* and installed
to file systems remains valid and we avoid problems on partial updates.

On i386/amd64, define two flag bits in the boot parameters which control
whether modules are loaded and whether boot.cfg is evaluated, and
set both to "off" per default in the PXE bootloader.
Extend "installboot" to toggle the bits.
This way, pxeboot works with existing dhcp server setups (and as
described in the manpage) out of the box. Also, boot.cfg reading
involves a stat() call which is horribly inefficient with the
TFTP pseudo file system.

Remove extra trailing slash in ${S} path.

Convert more MACHINE tests to MACHINE_ARCH

branches: 1.15.2; 1.15.8;
disable PIE for boot code.

branches: 1.14.40; 1.14.44; 1.14.50;
Install pxeboot_ia32.bin as part of a "make release" on i386 and amd64.

branches: 1.13.2; 1.13.4;
Fix ypo. (hi abs!)

Noticed at #NetBSD-code.

Enable EPIA_HACK here too - http://mail-index.netbsd.org/port-i386/2006/08/18/0002.html

branches: 1.11.4; 1.11.6;
Use LDFLAGS (with $(CC) syntax) instead of appending to LD.

branches: 1.10.4;
- -mcpu=i386 is gone in GCC4
- we need -Wno-attributes (to avoid __packed__ warnings)
- use -Wno-pointer-sign
- remove some redundancy from pxeboot/Makefile

branches: 1.9.6; 1.9.8; 1.9.10; 1.9.14;
Take the console (etc) parameters from the patchable header, instead of
hard-coded constants.
Now the updated installboot can be used to modify the console and baud rate.

copy some settings from ../boot/Makefile.boot that let us try to load
64-bit kernels on amd64.

branches: 1.7.10; 1.7.12;
fix several related bugs that cause sysctl machdep.diskinfo to
lose when booted from pxeboot.
. make sure that i386_alldisks gets initialized even if
bios geometry information is not available in the bootinfo
. if i386_alldisks is not initialized, have sysctl return EOPNOTSUPP
. compile pxeboot with -DPASS_BIOSGEOM and I386_INCLUDE_DISK=yes
this may increase the size of pxeboot which is required to run
in 64k. However, it seems to be working ok on my system

branches: 1.6.2; 1.6.4;
Add -DSLOW to reduce the size of libz a bit.

use our own NFS fs implementation, which is smaller, does a better job
in error reporting and coexists with the TFTP fs

use NOMAN instead of MKMAN=no. noted by David Bonnafous on port-i386@

branches: 1.3.2; 1.3.4; 1.3.16;
* Rename the pxeboot binary from "pxeboot" to "pxeboot_ia32", since
there are other architectures (e.g. IA64) which also use PXE.
* Get the console right in pxeboot, from Andreas Gustafsson.
* Add a "com0" version of pxeboot, to address the same issue that
biosboot_com0 addresses.

Cleanup of i386 bootloader building:
* Add separate directories for the individual netboot ROMs and
always build them.
* Each bootloader Makefile explicitly specifies the start file
and link address it wishes to use.
* genprom becomes a proper host program.
* Make sure all generated files get cleaned up.
* Set BINDIR in Makefile.inc
* Add compressed image support to pxeboot.
* Make pxeboot use the default serial console speed (9600).

Add a PXE boot loader. PXE interface code is derived by code
written by Alfred Perlstein, Paul Saab, John Baldwin for FreeBSD.

Still needs some work, but this will load a kernel on a Soekris net4501.

Many thanks to Frank van der Linden and his simulator wizardry for
helping me debug the low-level startup code.