kernel: Make KERNEL_MAKEOPTS recursively expanded

KERNEL_MAKEOPTS will get expanded when it is used and not when it is
defined in the kernel.mk file now. This fixes problems finding dependent
kernel modules when it is used by a kernel module package.

Without this change the build of packages which depend on other out of
tree modules failed when they used KERNEL_MAKE because some symbols could
not be found. This happened because KERNEL_MAKE_FLAGS which contains a
"if $(__package_mk)" was  evaluated where KERNEL_MAKEOPTS was defined
and not when the KERNEL_MAKE was used. For packages which included
kernel.mk before package.mk we saw this problem. One workaround
was to use the correct include order and the other one was to not
use KERNEL_MAKE_FLAGS, but copy its content.
Signed-off-by: NHauke Mehrtens <hmehrtens@maxlinear.com>
(cherry picked from commit 06ad3ade)

include/kernel.mk

@@ -126,7 +126,7 @@ ifeq ($(call qstrip,$(CONFIG_EXTERNAL_KERNEL_TREE))$(call qstrip,$(CONFIG_KERNEL
 	KERNELRELEASE=$(LINUX_VERSION)
 endif
 
-KERNEL_MAKEOPTS := -C $(LINUX_DIR) $(KERNEL_MAKE_FLAGS)
+KERNEL_MAKEOPTS = -C $(LINUX_DIR) $(KERNEL_MAKE_FLAGS)
 
 ifdef CONFIG_USE_SPARSE
   KERNEL_MAKEOPTS += C=1 CHECK=$(STAGING_DIR_HOST)/bin/sparse