crunchgen: fix NULL-deref bug introduced in r364647

While porting over the local changes from CheriBSD for upstreaming, I
accidentally committed a broken version of find_entry_point(): we have to
return NULL if the value is not found instead of a value with
ep->name == NULL, since the checks in main were changed to check ep instead
of ep->name for NULL.

This only matters if the crunched tool cannot be found using normal lookup
and one of the fallback paths is used, so it's unlikely to be triggered
in rescue. However, I noticed that one of our CheriBSD test scripts was
failing to run commands under su on minimal disk images where all
binaries are hardlinks to a cheribsdbox tool generated with crunchgen.

This also updates the bootstrapping check in Makefile.inc1 to bootstrap
crunchgen up to the next version bump.

Reviewed By: kevans
Differential Revision: https://reviews.freebsd.org/D27474