Allow objcopy to work without filling gaps with 0xff

This is currently done for all targets, since 0xff is the default erased
value for most flash devices. In some cases this is not what we want (e.g.
for EFI images) so provide a command to do a vanilla objcopy.

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