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]>