package/rpi-firmware: add option to install DTB overlays

Currently, Buildroot does not support building the overlays that are
bundled in the Linux kernel, so all we can do is install the ones
pre-built in rpi-firmware.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Floris Bos <bos@je-eigen-domein.nl>
Cc: Pascal de Bruijn <pmjdebruijn@pcode.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>