package/rpi-firmware: install DTB blobs

Add an option to install all the DTBs:
  - standard DTBs for standalon A/B and A+/B+ models;
  - overlay DTBs for the 'hats' addon boards.

Install the DTBs as per the traditional layout expected by all RPi
users, that is:
  - base DTBs alongside the other boot files;
  - overlay DTBs in a sub-directory.

This requires the user provide a specially configured Linux defconfig
file, as the default ones do not enable USE_OF.

[Thomas: adjust comment explaining why we use a different version when
installing the DTBs is selected.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>