efi_loader: Move to normal debug infrastructure

We introduced special "DEBUG_EFI" defines when the efi loader
support was new. After giving it a bit of thought, turns out
we really didn't have to - the normal #define DEBUG infrastructure
works well enough for efi loader as well.

So this patch switches to the common debug() and #define DEBUG
way of printing debug information.

Signed-off-by: Alexander Graf <agraf@suse.de>