flash: add stm32f2x async flash loader

flash: add stm32f2x async flash loader

This enable the stm32f2x flash driver to use the asynchronous
algorithm support.

Speed increase is as follows:
before - wrote 1048576 bytes from file stm32f4x.bin in 30.453804s (33.625 KiB/s)
after - wrote 1048576 bytes from file stm32f4x.bin in 23.679497s (43.244 KiB/s)

This also fixes a bug that was in the old flash loader.
The old loader waited while bit16 of the status reg was 0, the new
loader waits until this bit is 0 as stated in the flash spec.
Bizarrely this bug did not effect programming on any tested parts.

Change-Id: I3efc94d42cbe81283673a8f4203700638080af6e
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/460
Tested-by: jenkins