Replace outdated help message with a reference to README.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

Signed-off-by: Randy Dunlap <[email protected]>
Acked-by: Sam Ravnborg <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>