flash: cleanup stm32lx driver

Handle any leading bytes upto the next 128 byte page, enabling us to safely
use the faster page write.

Rather than use a separate word/byte write to program any trailing bytes
we use a combined write function.

Use memcpy for byte writes and change loader to using bytes.

Change-Id: Ie0164a30388f018dd00e752cf5ff87d4f96ced97
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1008
Tested-by: jenkins