efi_loader: add checking for incorrect use of EFI_ENTRY/EXIT

Missing an EFI_ENTRY() or doubling up EFI_EXIT() leads to non-obvious
crashes.  Let's add some error checking.

Signed-off-by: Rob Clark <robdclark@gmail.com>
[agraf: fix bogus assert() and fix app_gd breakage]
Signed-off-by: Alexander Graf <agraf@suse.de>