kbuild: update comment about KBUILD_ALLDIRS

Commit 000ec95f ("kbuild: pkg: rename scripts/package/Makefile to
scripts/Makefile.package") missed to update this comment.

Fixes: 000ec95f ("kbuild: pkg: rename scripts/package/Makefile to scripts/Makefile.package")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>