stm32 configs: use 4kB working area size by default

This is needed for configs that might be used with the cheapest
STM32F100 parts that have only 4kB SRAM.

Restrictions for the other STM32 families are verified to be set
appropriately.

Change-Id: I1ad2370435015604db9f27c1a76c153480311a28
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1378


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