boot1 generate-fat: generate all templates at once

In advance of other changes to the fat template generation process, have
generate-fat.sh create all template files at the same time so that they
cannot get out of sync.

Also correct a longstanding but where BOOT1_OFFSET was overwritten on
each invocation. A previous version of this patch stored a per-arch
offset (e.g. BOOT1_arm64_OFFSET) but that was deemed unnecessary.
Instead just hardcode the known offset that applies to all archs (0x2d)
and fail if the offset happens to be different.

Ongiong work (using newfs_msdos in bsdinstall and adding msdosfs support
to makefs) will eventually allow us to do away with this fat template
hack altogether, but in the near term we have a few improvements that
will build on this.

Reviewed by: allanjude, imp, Eric McCorkle
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10931