kinetis: support mass erase on boards without SRST

kinetis: support mass erase on boards without SRST

NXP (nee Freescale) documents the mass erase procedure using the MDM-AP
block in AN4835. Existing support for this feature did not properly
handle boards without SRST. This patch updates the mass_erase command
such that it works correctly on these boards. Additionally, the core is
left in a halted state once complete to prevent reset loops due to the
watchdog as reported by some users.

Since the MDM-AP provides an additional method of halting and resetting
the core that is disconnected from the DAP, additional commands are
provided to manage this state. These commands are particularly helpful
when connecting to a target with an unknown state.

Change-Id: I40f006d5d964befb12b019c5d509988decdd3f91
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/3540
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>