kbuild: improve comments on KBUILD_SRC

Original comments is confusing on "OBJ directory", make it clear.
Bonus: move comments close to what it wants to comment.

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>