build: pass KBUILD_EXTRA_SYMBOLS with symvers files for kernel module packages

This replaces the previous (deprecated) method of collecting symvers data
in $(PKG_BUILD_DIR)/Module.symvers, which does not work on newer kernels

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/kernel-build.mk

@@ -136,6 +136,7 @@ define BuildKernel
   $(LINUX_DIR)/.modules: export PKG_CONFIG_LIBDIR=$$(STAGING_DIR_HOST)/lib/pkgconfig
   $(LINUX_DIR)/.modules: $(STAMP_CONFIGURED) $(LINUX_DIR)/.config FORCE
 	$(Kernel/CompileModules)
+	mkdir -p $(PKG_SYMVERS_DIR)
 	touch $$@
 
   $(LINUX_DIR)/.image: export STAGING_PREFIX=$$(STAGING_DIR_HOST)

include/kernel.mk

@@ -118,7 +118,8 @@ KERNEL_MAKE_FLAGS = \
 	CONFIG_SHELL="$(BASH)" \
 	$(if $(findstring c,$(OPENWRT_VERBOSE)),V=1,V='') \
 	$(if $(PKG_BUILD_ID),LDFLAGS_MODULE=--build-id=0x$(PKG_BUILD_ID)) \
-	cmd_syscalls=
+	cmd_syscalls= \
+	$(if $(__package_mk),KBUILD_EXTRA_SYMBOLS="$(wildcard $(PKG_SYMVERS_DIR)/*.symvers)")
 
 ifeq ($(call qstrip,$(CONFIG_EXTERNAL_KERNEL_TREE))$(call qstrip,$(CONFIG_KERNEL_GIT_CLONE_URI)),)
   KERNEL_MAKE_FLAGS += \
@@ -140,14 +141,6 @@ PKG_EXTMOD_SUBDIRS ?= .
 
 PKG_SYMVERS_DIR = $(KERNEL_BUILD_DIR)/symvers
 
-define populate_module_symvers
-	@mkdir -p $(PKG_SYMVERS_DIR)
-	cat /dev/null > $(PKG_SYMVERS_DIR)/$(PKG_NAME).symvers; \
-	for subdir in $(PKG_EXTMOD_SUBDIRS); do \
-		cat $(PKG_SYMVERS_DIR)/*.symvers 2>/dev/null > $(PKG_BUILD_DIR)/$$$$subdir/Module.symvers; \
-	done
-endef
-
 define collect_module_symvers
 	for subdir in $(PKG_EXTMOD_SUBDIRS); do \
 		realdir=$$$$(readlink -f $(PKG_BUILD_DIR)); \
@@ -161,7 +154,6 @@ endef
 
 define KernelPackage/hooks
   ifneq ($(PKG_NAME),kernel)
-    Hooks/Compile/Pre += populate_module_symvers
     Hooks/Compile/Post += collect_module_symvers
   endif
   define KernelPackage/hooks