package/busybox: Add facility for DHCP hooks

The (u)dhcpc hook installed by the busybox package configures the
network and exits. If we want to do anything further with a DHCP lease,
we'd have to replace the script entirely.

This change introduces a .d directory for hooks (based on the script
filename), which are executed after the interface configuration. This
allows packages to drop a script file in the .d directory to perform
actions on DHCP events.

We'll use this in a later change to notify petitboot of DHCP boot
information.

[Thomas: update to latest Buildroot, fix indentation.]

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>