cortex_m: suggest using hardware srst if VECTRESET used

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>