buildroot

busybox: add comment that other packages are needed

Inspired on the confusion in [1], we want to warn the user that building
without busybox may not lead to a working system.

[1] https://bugs.busybox.net/show_bug.cgi?id=9526

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>