Overhaul xxboot. And merge floppy boot, taken from boot_ustar.
- Rewrite boot.S completely.
boot.S now supports boot from SCSI HD/CD and floppy.
- Use IOCS call to identify the floppy format, instead of chkfmt.s which
accesses hardware directly.
- Import print_hex() debug function from boot_ufs.
- Import a feature that displays initial registers (for debug) from boot_ufs,
and restore it (this in boot_ufs has been broken).
- Add size optimized alternatives for some libkern routines.
- Stop linking libsa to prevent to link unexpected objects.
- Bump version to 2.0.

Initialize the screen in all primary bootloaders.
- Some IPL (or boot selector) don't do it, but current secondary bootloader
expects the screen to be initialized to display logo.
To keep messages (like version signature) displayed by primary bootloader,
this initialization should be done early in all primary bootloaders.
- For boot_ustar.S, relocates some registers to save bootinfo (%d0).
- Bumps version.

Confirmed on X68030+060turbo and my X68030. This problem was found on
X68030+060turbo at NetBSD booth in OSC 2011 Hiroshima (8+ years ago).

branches: 1.2.2; 1.2.40; 1.2.46;
Make xxboot for ufs variants actually functional:
- set bootdev properly even on SCSI HD and floppy boot
per SDBOOT/FDBOOT/CDBOOT defines in each Makefile
- always print xxboot name and version
- use IOCS PRINT calls instead of printf to avoid size overflow in DEBUG case
- wrap debug stuff with #ifdef XXBOOT_DEBUG
- bump version to denote changes
Mostly taken from old boot_ufs.

XXX: xxboot_lfs[12] are untested.

branches: 1.1.2; 1.1.4;
Initial support for xxboot (cdboot_cd9660, specifically).