replace references to Busybox with BusyBox

The correct capitalised form appears to be "BusyBox" rather than "Busybox";
fix all references to the latter form. (Most such references occur in the
manual and in commentary in package makefiles.)

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>