bsdinstall: bootconfig: Try to clean old efi boot entries

If one install FreeBSD on the same machine multiple times in a row or
on different harddrive they have a lot of 'FreeBSD' efi boot entries added.
With this patch we now do :

• If there is no 'FreeBSD' entry we add one like before
• If there is one or more entries we ask the user if they want to delete them all and add a new one
  • If they say yes we do that
  • If they say no we prompt them an inputbox so they can enter a different entry name if they want, it defaults to 'FreeBSD'

Reviewed by: bapt, imp
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33330

(cherry picked from commit 40c928e7b80a7e2ba3652f7c3dc1d30fe92e5b48)