lib.sh: rm Ubuntu boot entry on EFI systems

lib.sh: rm Ubuntu boot entry on EFI systems

On EFI-enabled systems, grub-install from grub-efi-* package
installs a boot entry named "ubuntu".

MaaS relies on IPMI to set boot order to PXE first; however
on systems with buggy firmware or without full IPMI support,
that fails, leading to booting Ubuntu from hard disk instead.

Work around this by clearing any previous Ubuntu boot entry
from board flash, before starting a new baremetal deploy.

NOTE: This only runs against nodes that are online from a
previous deploy.

Closes: ARMBAND-47

Change-Id: I1c4ece09e42845ce2a1b7119ec69e46e5ca12376
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>