This makes use of the newly introduced usage field in the flash bank
structure.
Also remove the assertion if usage field is null and
lets print a DEBUG_LOG message instead.
Change-Id: I384bf0e2c444fcc99deef73aec9ef01149a91c76
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/391
Tested-by: jenkins