gdb: fix broken qCRC packet handling

gdb: fix broken qCRC packet handling

The rtos layer was incorrectly handling a qCRC packet as a qC packet.
Make sure we check for the qCRC packet and return unhandled so the gdb
server gets a chance to handle it.

This packet is used in the gdb compare-sections cmd.

Change-Id: I21f8e5fa7225fccd13d65cf9e40186895065a7e3
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/933
Tested-by: jenkins
Reviewed-by: Matthias Blaicher <matthias@blaicher.com>
Reviewed-by: Peter Stuge <peter@stuge.se>