build: cache kernel module package compiling

Kernel module packages compiling is not cached (e.g. mac80211)
even with CONFIG_CCACHE on.

CC should be set to KERNEL_CC in KERNEL_MAKE_FLAGS at kernel.mk
to allow kernel module packages using ccache.

Signed-off-by: Zeyu Dong <dzy201415@gmail.com>

include/kernel-defaults.mk

@@ -9,10 +9,6 @@ endif
 
 INITRAMFS_EXTRA_FILES ?= $(GENERIC_PLATFORM_DIR)/image/initramfs-base-files.txt
 
-ifneq (,$(KERNEL_CC))
-  KERNEL_MAKEOPTS += CC="$(KERNEL_CC)"
-endif
-
 export HOST_EXTRACFLAGS=-I$(STAGING_DIR_HOST)/include
 
 # defined in quilt.mk

include/kernel.mk

@@ -119,6 +119,10 @@ KERNEL_MAKE_FLAGS = \
 	cmd_syscalls= \
 	$(if $(__package_mk),KBUILD_EXTRA_SYMBOLS="$(wildcard $(PKG_SYMVERS_DIR)/*.symvers)")
 
+ifneq (,$(KERNEL_CC))
+  KERNEL_MAKE_FLAGS += CC="$(KERNEL_CC)"
+endif
+
 KERNEL_NOSTDINC_FLAGS = \
 	-nostdinc $(if $(DUMP),, -isystem $(shell $(TARGET_CC) -print-file-name=include))