Add debos recipe for raspberrypi ARM64-based boards

This adds a debos recipe for building a desktop image for a ARM64-based
RaspberryPi. It include a lightweight desktop, that is LXDE, create a
default user "pi" that has sudo access rights. As on Raspbian, the "pi"
user is automatically logged in via lightdm. The buster debian splash is
also automatically enabled

debos/overlays/lxde-pi/home/pi/.config/pcmanfm/LXDE/desktop-items-0.conf

@@ -0,0 +1,14 @@
+[*]
+wallpaper_mode=crop
+wallpaper_common=1
+wallpaper=/home/pi/.config/pcmanfm/LXDE/pi_white.jpg
+desktop_bg=#000000
+desktop_fg=#ffffff
+desktop_shadow=#000000
+desktop_font=Sans 12
+show_wm_menu=0
+sort=mtime;ascending;
+show_documents=0
+show_trash=0
+show_mounts=0
+