flash Kinetis: dynamic status detection before starting mass_erase

flash Kinetis: dynamic status detection before starting mass_erase

The change prevents starting mass_erase in unstable state of MCU
(RESET/WDOG loop).
mass_erase of secured MCU using manual reset button is supported.
Timeouts are measured by timeval_ms() instead of iteration count.
mass_erase timeout prolonged to 16 seconds because aborting
mass_erase in progress (deasserting reset) leaves the device
in security locked state.

Change-Id: I6605532df56080a54c2a1dfe49094e3db4ce534a
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3551
Tested-by: jenkins
Reviewed-by: Steven Stallion <stallion@squareup.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>