libcec: add libcec for HDMI device Control

This patch adds libcec for HDMI device control.  It enables Raspberry Pi
support if the rpi-userland package is installed.

[Thomas: add C++ dependency, reformat .mk header.]
Signed-off-by: Spenser Gilliland <spenser@gillilanding.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>