layers.txt: Initial setup for Tofee repository

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
bblayers.conf: add meta-acer layer

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>
setup-env: set MACHINE=a500 by default

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>