target: add function to get number of bytes available in working area

target: add function to get number of bytes available in working area

This is a much cleaner solution to the problem of allocating as much
working area as possible than what is currently being done in most/all flash
drivers (which is: try an arbitrary sized chunk, if it fails, pick a smaller
number, rinse and repeat).

Use this function to find out how much working area is available, limit or
restrict that amount at will and then simply allocate it.

Change-Id: Ib7d5d0b7485aed3e0a4fad60c1bedb7dfd16146f
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/446
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>