kbuild: small update of allnoconfig description

'allnoconfig' is described by 'make help' as a "minimal config", that's not
strictly correct. To be pedantic, a minimal config would be one where
EMBEDDED was set to Y and most things therein disabled etc. Simply
answering 'no' to all options does not give a minimal config.
A better description of allnoconfig is that it answers all options with 'no'.

This patch updates the description.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>