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.h

@@ -87,6 +87,10 @@ struct flash_bank {
 	int chip_width; /**< Width of the chip in bytes (1,2,4 bytes) */
 	int bus_width; /**< Maximum bus width, in bytes (1,2,4 bytes) */
 
+	/** Default padded value used, normally this matches the  flash
+	 * erased value. Defaults to 0xFF. */
+	uint8_t default_padded_value;
+
 	/**
 	 * The number of sectors on this chip.  This value will
 	 * be set intially to 0, and the flash driver must set this to