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>

include/common.h

@@ -59,9 +59,6 @@ void	hang		(void) __attribute__ ((noreturn));
 
 #include <display_options.h>
 
-/* common/cmd_source.c */
-int	source (ulong addr, const char *fit_uname);
-
 /* common/memsize.c */
 long	get_ram_size  (long *, long);
 phys_size_t get_effective_memsize(void);