buildroot

Makefile: make help: remove <package>- help, add link to online manual

We only documented a few of the <package>- targets and it's hard to
decide which ones are relevant for make help. Since the help is already
way too long, it's better to remove these advanced targets.

Instead, let's refer to the online manual.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>