configs: ti_armv7_keystone2: start using armv7_common

Try to maintain as much commonality by conditionally including stuff
in armv7_common as necessary and removing the common defines from
keystone2 header.

Note: as part of this change, all keystone2 platforms will now start
using the generic u-boot prompt instead of the custom prompt.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>