flash/nor/stm32l: fix mass erase

Topaz reports on http://sourceforge.net/p/openocd/tickets/87/

 that
protection level constants are mixed up. This leads to device ending
up in protection level 1 after mass erase.

Additional work is required to actually put the device in RDP Level 1
and then back to Level 0, as Option bootloader launch is a special
kind of full target reset.

To be able to flash properly after mass_erase a "reset init" is needed
(it's anyway recommended to always perform it before any flash
operation).

Change-Id: I9a838909458039bb0114d3019723bf134fa4d7c9
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2490


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>