target: rename cortex_m3 to cortex_m

Rename cortex_m3 target to use a more correct cortex_m name.
This also adds a deprecated_name var so that older scripts issue a warning
to update the target name.

cfg files have also been updated to the new target name.

Change-Id: Ia8429f38e88da677249c5caa560c50f8ce56ea10
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1129
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>