image: Move common image code to image_board and command

We should use the cmd/ directory for commands rather than for common code
used elsewhere in U-Boot. Move the common 'source' code into
image-board.c to achieve this.

The image_source_script() function needs to call run_command_list()  so
seems to belong better in the command library. Move and rename it.

Signed-off-by: Simon Glass <sjg@chromium.org>