bsdinstall: fix EFI boot entry creation

update_uefi_bootentry assumes that the caller sets FREEBSD_BOOTNAME and
mntpt, which isn't the case anymore. The result is that there is no
"FreeBSD" boot entry created/updated after install. Most machines manage
to boot from the removable media path (if the loader is installed there
too), but some don't.

Take the loader's path as an argument and rename the variable used in
the ZFS mirror loop so mntpt can be reused below.

Also mark nentries as a local variable so it doesn't leak out of the
function.

PR: 293385
Fixes: 494de51bc0074472d1b01604f085daea0844f240
MFC after: 2 days
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D55469

(cherry picked from commit 0fd91c489134643ac9e38c0f55ba7464fe892c5e)