add "%" prefix to register names in assembly code.
change assembly functions to return values in %a0 instead of %d0.
C symbols no longer prepend an underscore, adjust assembly code for this.
32-bit values are now 32-bit aligned instead of 16-bit aligned,
adjust structure packing and padding to override this where necessary.
make EXEC_ELF std, make EXEC_AOUT and COMPAT_AOUT_M68K optional.
use the MI loadfile() instead of several home-grown versions.
bootblock revision strings, use bootbock_name[] (i.e. "NetBSD/sun3")
in initial bootblock messages instead of just "NetBSD"
while here, do sligh Makefile cleanup to share more code between them and also
to make buildable on NetBSD 1.4.1 (my test compile system)
get rid of the XX define hack for netboot & ufsboot, build xxboot.c as
part of libsa - previos main() was renamed to xxboot_main(), main()
is now in respective conf.c files and calls xxboot_main() with
appropriate name of boot block type
XXX I had no chance to actually test the resulting bootblocks, but the
changes were fairly streightforward and should no influence functionality of
boot code
