sdk: fix GCC and Python dangling symlinks

Force prereq again in SDK in order to fix GCC and Python dangling
symlinks:

 staging_dir/host/bin/g++ -> /builder/ath79_generic/ccache_cxx.sh
 staging_dir/host/bin/gcc -> /builder/ath79_generic/ccache_cc.sh
 staging_dir/host/bin/python -> /usr/bin/python3.5
 staging_dir/host/bin/python3 -> /usr/bin/python3.5

Ref: FS#2424
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/sdk/Makefile

@@ -144,7 +144,8 @@ $(BIN_DIR)/$(SDK_NAME).tar.xz: clean
 	-rm -rf \
 		$(SDK_BUILD_DIR)/$(STAGING_SUBDIR_HOST)/ccache \
 		$(SDK_BUILD_DIR)/$(STAGING_SUBDIR_TARGET)/ccache \
-		$(SDK_BUILD_DIR)/$(STAGING_SUBDIR_TOOLCHAIN)/ccache
+		$(SDK_BUILD_DIR)/$(STAGING_SUBDIR_TOOLCHAIN)/ccache \
+		$(SDK_BUILD_DIR)/$(STAGING_SUBDIR_HOST)/.prereq-build
 
 	-rm -f $(SDK_BUILD_DIR)/feeds.conf.default
 	$(if $(BASE_FEED),echo "$(BASE_FEED)" > $(SDK_BUILD_DIR)/feeds.conf.default)