Add support for directly booting from FAT12 filesystems.
This shouldn't really be needed except that Linux wont
mount a small FAT16 filesystem - it always treats it as FAT12
even when you tell it otherwise.
There was enough free space before the main FS on the USB stick
I've been using for an extra small FAT fs to boot from.

branches: 1.2.2; 1.2.6; 1.2.86; 1.2.90;
Change build to allow a fat32 bootstrap to be built from the same sources
(the relevant changes haven't been done though).
Call built binary bootxx_fat16 for consistency.
Use 'push %cs' to push a zero value in places where we had relied on %bx
being zero from much higher up the code.

Add a sub-512 byte piece of code that can be written (by a modified installboot)
into the first sector of a FAT16 filesystem and will load 'boot' (and hence)
the netbsd kernel from it.
Unlike bootxx_msdosfs this doesn't need the filesystem to have 8k of 'reserved'
sectors, and may well work on non-512 byte media (as yet untested).