guacamayo-image.bbclass: customize RPi config.txt before doing rootfs

Pressently meta-raspberrypi generates the config as part of rpi-config
package, which means the config is same across all images. Since we need
to be able to specify config on per-image bases, we have to run sed
over it.
guacamayo-image-mex: specify memory layout and cpu clocking for RPi
gnome-settings-overrides: stop tracker from indexing removable media
guacamayo-session-egl: process xdg/autostart even on RPi
README.raspberrypi: update info on Mex status.