cortex_m: suggest using hardware srst if VECTRESET used

If the target does not support SYSRESETREQ we fall back to using VECTRESET.
This however does not reset the peripherals and we issue a warning to the user
to suggest using a reset-init script.

Also suggest that using hardware srst will give them the same functionality
as using SYSRESETREQ.

Change-Id: Ie1781c4b849fed66c52222e6539735537c879fb3
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/802
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

src/target/cortex_m.c

@@ -1025,7 +1025,7 @@ static int cortex_m3_assert_reset(struct target *target)
 
 		if (reset_config == CORTEX_M3_RESET_VECTRESET) {
 			LOG_WARNING("Only resetting the Cortex-M3 core, use a reset-init event "
-				"handler to reset any peripherals");
+				"handler to reset any peripherals or configure hardware srst support.");
 		}
 
 		{