Revert "build: allow simple build system customization with local.mk"

This reverts commit fe928c7a81810fd84210a5e9594b7fa8ce1064ce as it seems
to introduce build breakage when local.mk doesn't exist.

 make --trace
 toplevel.mk:216: target 'local.mk' does not exist

Signed-off-by: Petr Štetiar <[email protected]>

Makefile

@@ -103,5 +103,3 @@ world: prepare $(target/stamp-compile) $(package/stamp-compile) $(package/stamp-
 .PHONY: clean dirclean prereq prepare world package/symlinks package/symlinks-install package/symlinks-clean
 
 endif
-
--include local.mk