configs/imx6-sabreauto: Add a new defconfig

Introduce imx6-sabreauto_defconfig that uses mainline U-Boot and
mainline kernel.

As mainline U-Boot supports SPL, it is possible to boot the same
sdcard.img in the three mx6sabreauto variants: imx6q, imx6dl and imx6qp.

Also update the readme.txt file to explain how to build an image for
mx6sabreauto boards using mainline U-Boot and kernel.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
[Thomas: regenerate gitlab-ci.yml file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

configs/imx6-sabreauto_defconfig

@@ -0,0 +1,38 @@
+BR2_arm=y
+BR2_cortex_a9=y
+BR2_ARM_ENABLE_NEON=y
+BR2_ARM_ENABLE_VFP=y
+BR2_ARM_FPU_VFPV3=y
+
+# Linux headers same as kernel, a 4.13 series
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_13=y
+
+# System
+BR2_TARGET_GENERIC_GETTY_PORT="ttymxc3"
+
+# required tools to create the SD card image
+BR2_PACKAGE_HOST_DOSFSTOOLS=y
+BR2_PACKAGE_HOST_GENIMAGE=y
+BR2_PACKAGE_HOST_MTOOLS=y
+
+# Filesystem
+BR2_ROOTFS_POST_IMAGE_SCRIPT="board/freescale/common/imx/post-image.sh"
+BR2_TARGET_ROOTFS_EXT2=y
+BR2_TARGET_ROOTFS_EXT2_4=y
+
+# Bootloader
+BR2_TARGET_UBOOT=y
+BR2_TARGET_UBOOT_BOARDNAME="mx6sabreauto"
+BR2_TARGET_UBOOT_CUSTOM_VERSION=y
+BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2017.09"
+BR2_TARGET_UBOOT_FORMAT_IMG=y
+BR2_TARGET_UBOOT_SPL=y
+BR2_TARGET_UBOOT_SPL_NAME="SPL"
+
+# Kernel
+BR2_LINUX_KERNEL=y
+BR2_LINUX_KERNEL_CUSTOM_VERSION=y
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="4.13.3"
+BR2_LINUX_KERNEL_DEFCONFIG="imx_v6_v7"
+BR2_LINUX_KERNEL_DTS_SUPPORT=y
+BR2_LINUX_KERNEL_INTREE_DTS_NAME="imx6q-sabreauto imx6dl-sabreauto imx6qp-sabreauto"