busybox: move udhcp script from skeleton to package

Since udhcpc is part of busybox, it seems logical to move the udhcpc
script from skeleton to busybox.

[Peter: only install if not available in skeleton]
Signed-off-by: Lionel Landwerlin <llandwerlin@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>