Add a way to skip relocation

When running U-Boot as an EFI application we cannot relocate since we do not
have relocation information. U-Boot has already been relocated to a suitable
address.

Add a global_data flag to control skipping relocation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>