package/gnu-efi: fix legal-info

Fix a bunch of deficiencies:
  - 'debian/copyright' does not exist.
  - gnuefi is not GPLv2+. Only one AArch64 file has GPLv2+ headers,
    all others have BSD-3c
  - efilib is BSD-2c

Change the licensing information to include BSD-3c for the gnuefi parts,
and keep refernce to GPLv2+ as the Main README.gnuefi references it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>