realtek: add common definition of cameo based firmware

realtek: add common definition of cameo based firmware

The cameo-related recipes can also be used for APRESIA ApresiaLightGS
series devices. So create common definition for the devices manufactured
by Cameo.
And also, the model name of ApresiaLightGS120GT-SS is too long for cameo
header (max: 20 bytes), so use additional variable "CAMEO_BOARD_MODEL"
in Build/cameo-headers instead of DEVICE_MODEL to use the custom name.
(default of CAMEO_BOARD_MODEL: DEVICE_MODEL)

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>