docs/kbuild/makefiles: clean up indentation and whitespace

Remove the leading whitespaces, and clean up indentation and whitespace
in general.

Although the diff looks massive, it's trivial with 'diff -w' or 'git
show -w'.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>