flash/nor: Add erased_value to drivers and pass it to targets

flash/nor: Add erased_value to drivers and pass it to targets

struct flash_driver has a default_padded_value field that is similar,
but it can be changed by the user for the specific purpose of padding.

Add a new erased_value field and initialize it for all targets,
particularly stm32lx, xmc4xxx and virtual.

Use this value in core.c:default_flash_mem_blank_check(), the slow path.

Extend the target API to pass erased_value down to target code.
Adding an argument ensures that we catch all callers.

This allows us to merge xmc4xxx.c:xmc4xxx_blank_check_memory() into
armv7m:armv7m_blank_check_memory().

It further allows us to use default_flash_blank_check() in place of
xmc4xxx.c:xmc4xxx_flash_blank_check(), adding a potential slow path
fallback, as well as stm32lx:stm32lx_erase_check(), adding the potential
armv7m fast path with fallback to default_flash_mem_blank_check().

Fix a mips32 code comment while at it (zeroed -> erased).

The armv4_5 and mips32 target implementations will now error out if an
erase value other than 0xff is used, causing default_flash_blank_check()
to fall back to the default_flank_mem_blank_check() slow path.

Change-Id: I39323fbbc4b71c256cd567e439896d0245d4745f
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/3497
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>