efivar: add recipe

efivar is is a tool to manipulate UEFI variables.
This is obviously useful only for EFI machines that have
the efivarfs interface enabled in the kernel config (CONFIG_EFIVAR_FS),
and mounted (mount -t efivarfs efivarfs /sys/firmware/efi/efivars).

Sample usage:
GUID: 4a67b082-0a4c-41cf-b6c7-440b29bb8c4f
Name: "LoaderEntrySelected"
Attributes:
    Boot Service Access
    Runtime Service Access
Value:
00000000  62 00 6f 00 6f 00 74 00  00 00                    |b.o.o.t...      |

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>