cfg: add stm32 flash driver alias

This will enable the user to use either name when calling flash driver cmds.

For example the stm32f3x family use the same flash driver as the stm32f1x, so
the user has to use 'stm32f1x mass_erase 0' which can be confusing.
Now the user can also use 'stm32f3x mass_erase 0' with the same result.

Change-Id: I14280674c8dfa2ce1c01e327af7f38e36150462c
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1428
Tested-by: jenkins