efi_loader: identify PK, KEK, db, dbx correctly

To determine if a varible is on the of the authentication variables
PK, KEK, db, dbx we have to check both the name and the GUID.

Provide a function converting the variable-name/guid pair to an enum and
use it consistently.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>