bsdinstall: Improve message for existing FreeBSD EFI entry

When reinstalling FreeBSD bsdinstall reported "There are multiple
FreeBSD EFI boot entries." This sounds like something went wrong in the
past. Clarify that there may be only one existing entry, which is not
surprising for a reinstall.

Reviewed by: manu, ziaee
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51527