screen rotation and MAC detection via command line options

screen rotation runs xrandr -o ${ROTATE} if a boot command line option
concerto.rotate=${ROTATE} is passed in. ${ROTATE} can be normal,
left, right, or inverted. See xrandr man page.

MAC detection enables the Concerto 2 player to be used in a mode
compatible with Concerto v1. If the boot command line contains the
string "concerto.macdetect", a ?mac=xx:xx:xx:xx:xx:xx parameter
will be appended to the end of the concerto.url.