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>
(cherry picked from commit 68e672f3)

include/target.mk

@@ -173,22 +173,30 @@ USE_SUBTARGET_CONFIG = $(if $(wildcard $(LINUX_TARGET_CONFIG)),,$(if $(LINUX_SUB
 LINUX_RECONFIG_LIST = $(wildcard $(GENERIC_LINUX_CONFIG) $(LINUX_TARGET_CONFIG) $(if $(USE_SUBTARGET_CONFIG),$(LINUX_SUBTARGET_CONFIG)))
 LINUX_RECONFIG_TARGET = $(if $(USE_SUBTARGET_CONFIG),$(LINUX_SUBTARGET_CONFIG),$(LINUX_TARGET_CONFIG))
 
+CFG_TARGET = $(CONFIG_TARGET)
+ifeq ($(CFG_TARGET),platform)
+  CFG_TARGET = target
+  $(warning Deprecation warning: use CONFIG_TARGET=target instead.)
+else ifeq ($(CFG_TARGET),subtarget_platform)
+  CFG_TARGET = subtarget_target
+  $(warning Deprecation warning: use CONFIG_TARGET=subtarget_target instead.)
+endif
+
 # select the config file to be changed by kernel_menuconfig/kernel_oldconfig
-ifeq ($(CONFIG_TARGET),platform)
+ifeq ($(CFG_TARGET),target)
   LINUX_RECONFIG_LIST = $(wildcard $(GENERIC_LINUX_CONFIG) $(LINUX_TARGET_CONFIG))
   LINUX_RECONFIG_TARGET = $(LINUX_TARGET_CONFIG)
-endif
-ifeq ($(CONFIG_TARGET),subtarget)
+else ifeq ($(CFG_TARGET),subtarget)
   LINUX_RECONFIG_LIST = $(wildcard $(GENERIC_LINUX_CONFIG) $(LINUX_TARGET_CONFIG) $(LINUX_SUBTARGET_CONFIG))
   LINUX_RECONFIG_TARGET = $(LINUX_SUBTARGET_CONFIG)
-endif
-ifeq ($(CONFIG_TARGET),subtarget_platform)
+else ifeq ($(CFG_TARGET),subtarget_target)
   LINUX_RECONFIG_LIST = $(wildcard $(GENERIC_LINUX_CONFIG) $(LINUX_SUBTARGET_CONFIG) $(LINUX_TARGET_CONFIG))
   LINUX_RECONFIG_TARGET = $(LINUX_TARGET_CONFIG)
-endif
-ifeq ($(CONFIG_TARGET),env)
+else ifeq ($(CFG_TARGET),env)
   LINUX_RECONFIG_LIST = $(LINUX_KCONFIG_LIST)
   LINUX_RECONFIG_TARGET = $(TOPDIR)/env/kernel-config
+else ifneq ($(strip $(CFG_TARGET)),)
+  $(error CONFIG_TARGET=$(CFG_TARGET) is invalid. Valid: target|subtarget|subtarget_target|env)
 endif
 
 __linux_confcmd = $(2) $(patsubst %,+,$(wordlist 2,9999,$(1))) $(1)