lib.sh: Cleanup previous /boot/efi contents

lib.sh: Cleanup previous /boot/efi contents

Some UEFI firmwares insist on scanning removable drives, even when
boot entries were deleted from UEFI boot list (board flash).

To work around this, remove contents of </boot/efi/*>, so scanning
won't identify any valid EFI binaries.
Another option would be erasing partition tables, but identifying
the underlying disk(s) is more complicated, especially when using
LVM/RAID etc.

Change-Id: I9949b99b139b1642e3bd8f04de3bd5ef74d1ecc5
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>