kernel-install: prefer /boot over /boot/efi for $BOOT_ROOT

This restores the preference order from before 9e82a74c. The code
previous to that change 'preferred' /boot over /boot/efi; that
commit changed it to check /boot/efi before checking /boot.
Changing this precedence could (and did, for me) have unexpected
effects - it seems safer to leave it how it was.

Signed-off-by: Adam Williamson <awilliam@redhat.com>
(cherry picked from commit a5307e17)