nrf51 - Add async loader. Performance on nrf51822QAA/stlink-v2 from ~3.5KiB/s to...

nrf51 - Add async loader. Performance on nrf51822QAA/stlink-v2 from ~3.5KiB/s to ~19.5KiB/s.

Change-Id: Ib0bf41a0cec85f0bd5728551f8ad7f6255e4ea04
Signed-off-by: Angus Gratton <gus@projectgus.com>
[spamjunkeater@gmail.com: Cleanup buffer allocation, detect -1 for unknown pages]
Signed-off-by: Erdem U. Altunyurt <spamjunkeater@gmail.com>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2204
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>