build: target: improve UX of CONFIG_TARGET handling

build: target: improve UX of CONFIG_TARGET handling

Make it clear, that for `make kernel_{menu,old}config` it's possible to
use only following values for CONFIG_TARGET variable:

 * env
 * target
 * subtarget
 * subtarget_target

This should prevent misuse like `make kernel_menuconfig
CONFIG_TARGET=bcm2710` etc.

Keep support for obsolete `platform` and `subtarget_platform` targets
with deprecation notice so this compat stuff could be removed in the
future.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Acked-by: Piotr Dymacz <pepe2k@gmail.com>