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>

board/raspberrypi/readme.txt

@@ -60,7 +60,8 @@ After building, you should obtain this tree:
     |   +-- cmdline.txt
     |   +-- config.txt
     |   +-- fixup.dat
-    |   `-- start.elf
+    |   +-- start.elf
+    |   `-- overlays/               [3]
     +-- sdcard.img
     `-- zImage
 
@@ -69,6 +70,11 @@ After building, you should obtain this tree:
 
 [2] This is the mkknlimg DT-marked kernel.
 
+[3] Only for the Raspberry Pi 3 Model (overlay pi3-miniuart-bt is needed
+    to enable the RPi3 serial console otherwise occupied by the bluetooth
+    chip). Alternative would be to disable the serial console in cmdline.txt
+    and /etc/inittab.
+
 How to write the SD card
 ========================