Fix flash writing on stm32l0

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>