hla: support setting DCB_DEMCR on resume

This is only minimal support to enable use to catch a Hard Fault in
the stm32l flash bootloader.

Change-Id: I21d6a11893e2f1d173ebff1a651d6f52bf6eec32
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1103
Reviewed-by: Peter Stuge <peter@stuge.se>
Tested-by: jenkins