Fix flash writing on stm32l0

Fix "couldn't use loader, falling back to page memory writes" error on
stm32l0 which was caused by the use of cortex-m3 instructions in the
flash loader code. The loader is rewritten using cortex-m0 compatible
instructions

Signed-off-by: Armin van der Togt <armin@otheruse.nl>
Change-Id: If23027b8e09f74e45129e1f8452a04bb994c424e
Reviewed-on: http://openocd.zylin.com/4036


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>