target: add target async algorithm support

target: add target async algorithm support

Currently the stm32f1x flash driver uses an asynchronous algorithm
as part of the block flash programming. This greatly speeds up flash
programming as the target is always running.

Moving the async code to the target enable other targets to use this
added functionality.

Change-Id: I8e53f094c2ef7848a7f86ddb9a35b6edbfc8454a
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/402
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>