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
