The existing abcksum() also violates strict-aliasing rule
(while current gcc48 doesn't warn it) and fixing all violations
strictly requires whole reorganization of boot sector structures.
But it won't happen soon and this MD installboot should be integrated
into MI installboot(8) in future, and it requires whole overhaul anyway.
See long discussion in source-changes-d@ for details.
Should be pulled up to netbsd-7 if we switches m68k to using gcc48.
7839e5ad38