Makefile.inc1: Reinstate building sbin/md5 for BOOTSTRAP_ALL_TOOLS

The sha256 and sha512 binaries are used when building release images.
Bootstrapping them used to be gated on MK_BOOT, which wasn't quite right
but in practice worked, at least in our use cases downstream. Add back
bootstrapping sbin/md5 and its aliases for this purpose to fix building
release images on Linux and macOS.

Fixes: f213da893ca8 ("Makefile.inc1: Remove beri straggler")