efi_loader: avoid initializer element is not constant

When building with -pedantic the current definition of EFI_GUID() causes
an error 'initializer element is not constant'.

Currently EFI_GUID() is used both as an anonymous constant and as an
intializer. A conversion to efi_guid_t is not allowable when using
EFI_GUID() as an initializer. But it is needed when using it as an
anonymous constant.

We should not use EFI_GUID() for anything but an initializer. So let's
introduce a variable where needed and remove the conversion.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>