add missing PDF document under doc/

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
add description (in config file) note on how these devices relate

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
remove "su -" line from build.sh

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
add comment in prepare.sh

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>