flash/nor/kinetis: fix FCF handling

flash/nor/kinetis: fix FCF handling

Kinetis Flash Configuration Field needs special handling to prevent unwanted
locking of the device. Warn user about any difference between generated
FCF and FCF data in the programmed file. Inform user that re-programming
of already programmed FCF may fail on devices with FTFE flash module.

While on it remove useless setting of is_erased flag after erase.

Change-Id: I3911f436674547fa12ef3886c7d5e8cd889f9e2b
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Fixes: https://sourceforge.net/p/openocd/tickets/270/
Reported-by: Noel Diviney <vk3avm@users.sourceforge.net>
Reviewed-on: http://openocd.zylin.com/5753
Tested-by: jenkins