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>
diff --git a/drivers/usb/gadget/f_sdp.c b/drivers/usb/gadget/f_sdp.c
index 5ae5b62..9ea43f2 100644
--- a/drivers/usb/gadget/f_sdp.c
+++ b/drivers/usb/gadget/f_sdp.c
@@ -868,7 +868,7 @@
 			jump_to_image_no_args(&spl_image);
 #else
 			/* In U-Boot, allow jumps to scripts */
-			image_source_script(sdp_func->jmp_address, NULL, NULL);
+			cmd_source_script(sdp_func->jmp_address, NULL, NULL);
 #endif
 		}