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>

configs/raspberrypi3_defconfig

@@ -0,0 +1,36 @@
+BR2_arm=y
+BR2_cortex_a7=y
+BR2_ARM_EABIHF=y
+BR2_ARM_FPU_NEON_VFPV4=y
+
+BR2_TOOLCHAIN_BUILDROOT_CXX=y
+
+BR2_SYSTEM_DHCP="eth0"
+
+# Linux headers same as kernel, a 4.1 series
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_1=y
+
+BR2_LINUX_KERNEL=y
+BR2_LINUX_KERNEL_CUSTOM_GIT=y
+BR2_LINUX_KERNEL_CUSTOM_REPO_URL="https://github.com/raspberrypi/linux.git"
+BR2_LINUX_KERNEL_CUSTOM_REPO_VERSION="20fe468af4bb40fec0f81753da4b20a8bfc259c9"
+BR2_LINUX_KERNEL_DEFCONFIG="bcm2709"
+
+# Build the DTB from the kernel sources
+BR2_LINUX_KERNEL_DTS_SUPPORT=y
+BR2_LINUX_KERNEL_INTREE_DTS_NAME="bcm2710-rpi-3-b"
+
+BR2_PACKAGE_RPI_FIRMWARE=y
+# BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTB_OVERLAYS is not set
+
+# Required tools to create the SD image
+BR2_PACKAGE_HOST_DOSFSTOOLS=y
+BR2_PACKAGE_HOST_GENIMAGE=y
+BR2_PACKAGE_HOST_MTOOLS=y
+
+# Filesystem / image
+BR2_TARGET_ROOTFS_EXT2=y
+BR2_TARGET_ROOTFS_EXT2_4=y
+# BR2_TARGET_ROOTFS_TAR is not set
+BR2_ROOTFS_POST_BUILD_SCRIPT="board/raspberrypi3/post-build.sh"
+BR2_ROOTFS_POST_IMAGE_SCRIPT="board/raspberrypi3/post-image.sh"