keystone2: Move target selection to Kconfig

The config option TARGET_K2x_EVM is set by the k2x defconfigs to pick
a board target, but the header configs also set K2x_EVM. This config
is redundant, remove it and use TARGET_K2x_EVM everywhere in its place.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>