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/virtual.c

@@ -46,6 +46,7 @@ static void virtual_update_bank_info(struct flash_bank *bank)
 	bank->size = master_bank->size;
 	bank->chip_width = master_bank->chip_width;
 	bank->bus_width = master_bank->bus_width;
+	bank->default_padded_value = master_bank->default_padded_value;
 	bank->num_sectors = master_bank->num_sectors;
 	bank->sectors = master_bank->sectors;
 }