kernel-install: prefer /boot over /boot/efi for $BOOT_ROOT
    
    This restores the preference order from before 9e82a74. 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 a5307e173bf86d695fe85b8e15e91126e8618a14)