build: fix external module symbol collection if build_dir is a symlink

e26ffb31df fixed only embedded modules
symbol collection. If we are building external modules, like broadcom-wl
or lantiq dsl stuff then modules which do EXPORT_SYMBOL have unresolved
paths in Module.symvers and external module which depend on other
external modules will have empty dependencies, leading to broken
module loading.
This was discussed on IRC with Jonas some time ago.
Fix this by handling both resolved and unresolved paths.

Fixes: e26ffb31df ("build: fix module symbol collection if build_dir is a symlink")
Signed-off-by: Roman Yeryomin <roman@advem.lv>
[jonas.gorski@gmail.com: add appropriate fixes tag]
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>

include/kernel.mk

@@ -141,7 +141,10 @@ endef
 
 define collect_module_symvers
 	for subdir in $(PKG_EXTMOD_SUBDIRS); do \
-		grep -F $$$$(readlink -f $(PKG_BUILD_DIR)) $(PKG_BUILD_DIR)/$$$$subdir/Module.symvers >> $(PKG_BUILD_DIR)/Module.symvers.tmp; \
+		realdir=$$$$(readlink -f $(PKG_BUILD_DIR)); \
+		grep -F $(PKG_BUILD_DIR) $(PKG_BUILD_DIR)/$$$$subdir/Module.symvers >> $(PKG_BUILD_DIR)/Module.symvers.tmp; \
+		[ "$(PKG_BUILD_DIR)" = "$$$$realdir" ] || \
+			grep -F $$$$realdir $(PKG_BUILD_DIR)/$$$$subdir/Module.symvers >> $(PKG_BUILD_DIR)/Module.symvers.tmp; \
 	done; \
 	sort -u $(PKG_BUILD_DIR)/Module.symvers.tmp > $(PKG_BUILD_DIR)/Module.symvers; \
 	mv $(PKG_BUILD_DIR)/Module.symvers $(PKG_INFO_DIR)/$(PKG_NAME).symvers