common: Rename and move source()

This function has a very generic name which does not adequately describe
its purpose. Rename it and move it to image.h, since it relates to reading
a script from an image.

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 bcd1c5d..50836db 100644
--- a/drivers/usb/gadget/f_sdp.c
+++ b/drivers/usb/gadget/f_sdp.c
@@ -726,7 +726,7 @@
 			jump_to_image_no_args(&spl_image);
 #else
 			/* In U-Boot, allow jumps to scripts */
-			source(sdp_func->jmp_address, "script@1");
+			image_source_script(sdp_func->jmp_address, "script@1");
 #endif
 		}