Add interrupt support for emulator

This provides us a way to inject interrupts during a test. If a test has
interrupt_generator() defined, it will run in a separate thread. The
generator can then trigger interrupts when it decides to. The current
running task is suspended while emulator is executing ISR.

Also fixes a bug that tasks run without scheduler notifying them during
emulator start-up.

BUG=chrome-os-partner:19235
TEST=Repeatedly run all tests.
BRANCH=None

Change-Id: I0f921c47c0f848a9626da6272d9040e2b7c5ac86
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/55671

test/host_command.c

@@ -169,6 +169,7 @@ static int test_hostcmd_invalid_checksum(void)
 
 void run_test(void)
 {
+	wait_for_task_started();
 	test_reset();
 
 	RUN_TEST(test_hostcmd_ok);