efivars: Move pstore code into the new EFI directory

efivars: Move pstore code into the new EFI directory

efivars.c has grown far too large and needs to be divided up. Create a
new directory and move the persistence storage code to efi-pstore.c now
that it uses the new efivar API. This helps us to greatly reduce the
size of efivars.c and paves the way for moving other code out of
efivars.c.

Note that because CONFIG_EFI_VARS can be built as a module efi-pstore
must also include support for building as a module.

Reviewed-by: Tom Gundersen <[email protected]>
Tested-by: Tom Gundersen <[email protected]>
Cc: Seiji Aguchi <[email protected]>
Cc: Anton Vorontsov <[email protected]>
Cc: Colin Cross <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Matthew Garrett <[email protected]>
Cc: Tony Luck <[email protected]>
Signed-off-by: Matt Fleming <[email protected]>