stm32: add mass erase support for STM32L

The mass erase for STM32L was lack because the procedure is more complex
than the procedure for the STM32F4xx.

The reference manual RM0038 (L100 subfamily) page 79 is more accurate
than the reference manual for the STM32L0xx. On the L0, the mass-erase
erase also the EEPROM. This is a limit to mass erase on L0.

The mass erase procedure is a command of telnet interface.

Tested on Discovery L053 and Discovery L100.

Change-Id: I6a1d7a3669789aea89c59a006ab2d883f3d827ca
Signed-off-by: Rémi PRUD'HOMME <prudhomme.remi@gmail.com>
Reviewed-on: http://openocd.zylin.com/2319
Tested-by: jenkins
Reviewed-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>