configs/raspberrypi0w_defconfig: new defconfig for Raspberry Pi Zero W

Up to now, Raspberry Pi Zero W (rpi0w) could be built with the normal
raspberrypi0_defconfig. However, then you don't have support for the
Bluetooth out-of-the-box, which makes using a W a bit pointless.

Therefore, create a separate defconfig for the W. It is a copy of
raspberrypi0_defconfig with the following changes:
 - Add DT overlays (from rpi-firmware) to support Bluetooth.
 - Pass --add-pi3-miniuart-bt-overlay to post-image.sh.

Since there is now a separate raspberrypi0w_defconfig, the W support
can be removed from raspberrypi0_defconfig.

Signed-off-by: Yves Deweerdt <yves.deweerdt.linux@gmail.com>
[Arnout:
 - Bump to same kernel version as raspberrypi0.
 - Remove redundant comment.
 - Remove redundant default BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTB_OVERLAYS=y.
 - Improve commit log.
 - Refresh .gitlab-ci.yml.
]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

board/raspberrypi/genimage-raspberrypi0.cfg

@@ -2,7 +2,6 @@ image boot.vfat {
   vfat {
     files = {
       "bcm2708-rpi-b-plus.dtb",
-      "bcm2708-rpi-0-w.dtb",
       "rpi-firmware/bootcode.bin",
       "rpi-firmware/cmdline.txt",
       "rpi-firmware/config.txt",