include: introduce subtarget_platform CONFIG_TARGET

Sometimes it is useful to change platform's kernel config file but with loaded
subtarget's kernel config. Good example for this use case is malta.

Example:

$ make kernel_menuconfig CONFIG_TARGET=subtarget_platform

Signed-off-by: Luka Perkov <luka@openwrt.org>

SVN-Revision: 37818

include/target.mk

@@ -141,6 +141,10 @@ ifeq ($(CONFIG_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)
+  LINUX_RECONFIG_LIST = $(wildcard $(GENERIC_LINUX_CONFIG) $(LINUX_SUBTARGET_CONFIG) $(LINUX_TARGET_CONFIG))
+  LINUX_RECONFIG_TARGET = $(LINUX_TARGET_CONFIG)
+endif
 ifeq ($(CONFIG_TARGET),env)
   LINUX_RECONFIG_LIST = $(LINUX_KCONFIG_LIST)
   LINUX_RECONFIG_TARGET = $(TOPDIR)/env/kernel-config