target: rename cortex_a8 to cortex_a

Rename cortex_a8 target to use a more correct cortex_a 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: I0eb1429c9281321efeb444b27a662a941a2ab67f
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1130
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>