configs/chromebook_elm: new defconfig

configs/chromebook_elm: new defconfig

This commit adds supports for building buildroot kernel + rootfs
for MT8173 Elm board, also known as Chromebook Elm
(https://www.acer.com/ac/en/US/content/series/acerchromebookr13).

Though Chrome-OS is officially supproted on this board, the
mainline kernel works as well (benchmarks + conformance), and so
the 5.9 kernel is used. As the 5.9 kernel isn't yet released, we
use the 5.9-rc5 for now, which will be up-revd to 5.9 once its
released.

Using the mainline kernel means that we have to apply certain patches
to get the HDMI screen working. These patches are lying in the
"drm-misc-next" list and will make it to the kernel after 5.9. At that
time, we will remove the patches and point Buildroot to use the latest
kernel (hopefully, 5.10).

This commit also adds an ITS file (for creating FIT images), an ARGS
file (for providing kernel args) and a "sign.sh" script to generate
signed kernel images. Though the "sign.sh" is very similar to the
coresponding file under board/chromebook/snow, it cannot be shared
between both boards, as the script requires access to the board
specific its / args file.

Additionally a readme & defconfig is added to help the user get started.

Signed-off-by: Bilal Wasim <bilalwasim676@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>