lib/efi: Cleanup read/write routines

Make a new stucture struct efi_data to hold the info that describes
an efi variable.  Make a common routine efi_open that opens the efi
variable file.  Switch the efi get/set/del routines over to use
efi_open.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>