Add usage info for the Kbuild environment variable KBUILD_KCONFIG.
Signed-off-by: NRandy Dunlap <rdunlap@infradead.org>
Reviewed-by: NCao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>