In loops that typically take longer time to complete, check if there is
a pending shutdown request. If so, terminate the loop.
This allows to respond to a signal requesting a shutdown during some
loops which do not return control to main OpenOCD loop.
Change-Id: Iace0b58eddde1237832d0f9333a7c7b930565674
Signed-off-by: Evgeniy Naydanov <evgeniy.naydanov@syntacore.com>