configs/raspberrypi: update the DT-enabled config

Update to the latest stable kernel version; build an in-tree DTB; use
a in-tree defconfig; enable C++ (to match what we have in the non-DT
variant); do not install DTB overlays (this minimalist config does not
make use of them).

Generate the DTBs for both the "original" variant (A and B) and the
"Plus" variant (A+ and B+).

Drop our custom linux defconfig file now we use an in-tree one.

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>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>