flash: stm32f1x: write option bytes using the loader

flash: stm32f1x: write option bytes using the loader

Some debuggers (stlink) can't issue 16 bit writes and have to use a
loader to write flash memory.

Currently the loader is not used for option bytes, causing
stm32x_write_options to fail silently on such hardware.

Fix this by using stm32x_write_block to write option bytes as well.

Change-Id: I49c29d53ab5e162463cb349d4c89bef96467e587
Signed-off-by: Szymon Modzelewski <szmodzelewski@gmail.com>
Reviewed-on: http://openocd.zylin.com/480
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>