configs: add Raspberry Pi 3 defconfig

So far identical to the rpi2 one except for the dts file as the SW runs in
32bit mode.

[Peter: extend commit message]
Signed-off-by: Ubaldo Porcheddu <ubaldo@eja.it>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/raspberrypi/genimage-raspberrypi3.cfg

@@ -0,0 +1,30 @@
+image boot.vfat {
+  vfat {
+    files = {
+      "bcm2710-rpi-3-b.dtb",
+      "rpi-firmware/bootcode.bin",
+      "rpi-firmware/cmdline.txt",
+      "rpi-firmware/config.txt",
+      "rpi-firmware/fixup.dat",
+      "rpi-firmware/start.elf",
+      "kernel-marked/zImage"
+    }
+  }
+  size = 32M
+}
+
+image sdcard.img {
+  hdimage {
+  }
+
+  partition boot {
+    partition-type = 0xC
+    bootable = "true"
+    image = "boot.vfat"
+  }
+
+  partition rootfs {
+    partition-type = 0x83
+    image = "rootfs.ext4"
+  }
+}