raspberrypi3: fix serial console (load pi3-miniuart-bt overlay)
- enable BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTB_OVERLAYS in raspberrypi3_defconfig
- add copy of rpi-firmware/overlays directory to boot partition in genimage-raspberrypi3.cfg
- enhance post-image.sh script to add 'dtoverlay=pi3-miniuart-bt' on request
- add BR2_ROOTFS_POST_SCRIPT_ARGS="--add-pi3-miniuart-bt-overlay" in raspberrypi3_defconfig
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>