flash: add padded_value cmd

This adds support for setting the default value used when padding image
sections. Previously 0xff was used but some targets have an erased value
of 0x00.

Change-Id: If3df6fea3abf95b09daa3ff7be362acf991070ce
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1635
Tested-by: jenkins

src/flash/nor/core.c

@@ -710,7 +710,7 @@ int flash_write_unlock(struct target *target, struct image *image,
 
 			/* see if we need to pad the section */
 			while (padding[section]--)
-				(buffer + buffer_size)[size_read++] = 0xff;
+				(buffer + buffer_size)[size_read++] = c->default_padded_value;
 
 			buffer_size += size_read;
 			section_offset += size_read;