kbuild: document 'make prepare' in 'make help'

The output of 'make help' covers a lot of options, but doesn't include
a listing for 'make prepare'.  Here's a one-liner to fix that...

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>