Rather than open-coding EFI_EXIT() + callback + EFI_ENTRY(), introduce
an EFI_CALL() macro.  This makes callbacks into UEFI world (of which
there will be more in the future) more concise and easier to locate in
the code.
Signed-off-by: NRob Clark <robdclark@gmail.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>