fwk: Race condition in single-threaded mode

The main thread loop is checking the ISR event queue
for fwk_slist_is_empty(). The queue is being updated by an
ISR while it is being checked leading to an inconsistent
state.

Change-Id: Ice57ee96c6a91fdcb1e45be3a964c0dae6ed401c
Signed-off-by: Jim Quigley <jim.quigley@arm.com>