add git subtree

this is a locally modified version that properly handle tags
Squashed 'buildroot/' content from commit 15ace1a

git-subtree-dir: buildroot
git-subtree-split: 15ace1a845c9e7fc65b648bbaf4dd14e03d938fd
Merge commit '37ec663b101805b705375eafb64e884d485ecb7d' as 'buildroot'
configs: add RaspberryPi defconfig

This is a default configuration for the RaspberryPi, tested and functionnal.

The kernel used is the one developped by the RaspberryPi foundation as it's not
fully supported currently in mainline kernel.
The configuration used for the kernel is the default bcmrpi.

For more info, please find board/raspberrypi/README.txt

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
gestion du serveur ssh
gestion de pulseaudio
support partiel du wifi

il faut encor configurer /etc/network/interfaces et wpa_supplicant