configs/raspberrypi_dt: update and fix our defconfig

Lock to 3.18 headers; add comment about installing DTBs;
bump kernel sha1.

Reported-by: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/raspberrypi_dt_defconfig

@@ -1,12 +1,22 @@
 BR2_arm=y
 BR2_arm1176jzf_s=y
+
 BR2_TARGET_GENERIC_GETTY_PORT="tty1"
+
+# Lock to 3.18 headers as the RPi kernel with
+# DT support is based off the 3.18 branch
+BR2_KERNEL_HEADERS_VERSION=y
+BR2_DEFAULT_KERNEL_VERSION="3.18"
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_3_18=y
+
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_GIT=y
 BR2_LINUX_KERNEL_CUSTOM_REPO_URL="git://github.com/raspberrypi/linux.git"
-BR2_LINUX_KERNEL_CUSTOM_REPO_VERSION="03d0d9c73c17f74a7afa64ed9f022565e89b8dd5"
+BR2_LINUX_KERNEL_CUSTOM_REPO_VERSION="246530ff8a4f302b8666c6d9cf509407d8c1257a"
 BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
 BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="board/raspberrypi/linux-3.18-rpi-dt.defconfig"
 BR2_LINUX_KERNEL_ZIMAGE=y
+
+# Install the DTB files
 BR2_PACKAGE_RPI_FIRMWARE=y
 BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTBS=y