doc: Bring in Makefile documentation

U-Boot uses the Linux Kbuild build system. Add the associated
documentation so that people can understand the Makefiles better.

This is taken from Linux v5.12
Signed-off-by: NSimon Glass <sjg@chromium.org>

doc/develop/index.rst

@@ -13,6 +13,7 @@ Implementation
    driver-model/index
    global_data
    logging
+   makefiles
    menus
    uefi/index
    version