stm32f1x: fix bug in flash loader and restrict instruction set to armv6-m

stm32f1x: fix bug in flash loader and restrict instruction set to armv6-m

Correct the offset to the read pointer when clearing it on error.

Also restrict the instruction set to armv6-m so the flash driver can be
used on Cortex-M0 parts with the same flash controller.

Change-Id: I380f9dabcc41fb6e4d43a7e02f355e2381913f39
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/399
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-by: Jonathan Dumaresq <jdumaresq@cimeq.qc.ca>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>