efi: Create a function to set up for running EFI code

There is still duplicated code in efi_loader for tests and normal
operation.

Add a new bootefi_run_prepare() function which holds common code used to
set up U-Boot to run EFI code. Make use of this from the existing
bootefi_test_prepare() function, as well as do_bootefi_exec().

Also shorten a few variable names.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>