lib/efi: Add new routines to access efi variables

Provide methods to load/store petitboot's configuration on efi-based
platforms. A test case is also provided.

Signed-off-by: Ge Song <ge.song@hxt-semitech.com>
[Cleanup file comments, make efivarfs_path static.]
Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>