build: use config.site generated by autoconf-lean, drop hardcoded sitefiles

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

Makefile

@@ -79,12 +79,6 @@ endif
 
 # check prerequisites before starting to build
 prereq: $(target/stamp-prereq) tmp/.prereq_packages
-	@if [ ! -f "$(INCLUDE_DIR)/site/$(ARCH)" ]; then \
-		echo 'ERROR: Missing site config for architecture "$(ARCH)" !'; \
-		echo '       The missing file will cause configure scripts to fail during compilation.'; \
-		echo '       Please provide a "$(INCLUDE_DIR)/site/$(ARCH)" file and restart the build.'; \
-		exit 1; \
-	fi
 
 checksum: FORCE
 	$(call sha256sums,$(BIN_DIR),$(CONFIG_BUILDBOT))