[PATCH] config: update usage/help info

Replace outdated help message with a reference to README.  Update README
for make *config variants and environment variable info.

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>