efi_loader: indent entry/exit prints to show nesting level

This should make it easier to see when a callback back to UEFI world
calls back in to the u-boot world, and generally match up EFI_ENTRY()
and EFI_EXIT() calls.

Signed-off-by: Rob Clark <robdclark@gmail.com>
[agraf: remove static from const var]
Signed-off-by: Alexander Graf <agraf@suse.de>