efi_loader: Fix efi_exit gd clobbering

Commit f494950b (efi_loader: call __efi_exit_check in efi_exit) added a call
to __efi_exit_check inside efi_exit to account for the fact that we're exiting
the efi_exit function via a longjmp call.

However, __efi_exit_check also swizzles gd to the application gd while the
longjmp will put us back into EFI context, so we need the efi (u-boot) gd.

This patch fixes that up by explicitly setting gd back to efi_gd before
doing the longjmp. It also adds a few comments on why it does that.

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