board: add nvidia jetson tx2 support

Tested-by: Asaf Kahlon <asafka7@gmail.com>
Signed-off-by: Christian Stewart <christian@paral.in>

board/jetson/tx2/readme.txt

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+NVIDIA Jetson TX2
+
+Intro
+=====
+
+This configuration supports the Jetson TX2 devkit.
+
+Building
+========
+
+Configure Buildroot
+-------------------
+
+For Jetson TX2:
+
+  $ make jetsontx2_defconfig
+
+Build the rootfs
+----------------
+
+You may now build your rootfs with:
+
+  $ make
+
+
+Flashing
+========
+
+Once the build process is finished you will have the target binaries in the
+output/images directory, with a copy of linux4tegra.
+
+Flashing to the internal eMMC is done by booting to the official recovery mode,
+and flashing the system from there. The default factory-flashed TX2 is suitable.
+
+There are a lot of cases where the TX2 will not boot properly unless all of the
+peripherals are fully disconnected, power is disconnected, everything fully
+resets, and then the power is introduced back again.
+
+The recovery mode of the Jetson is used to flash. Entering recovery:
+
+ - Start with the machine powered off + fully unplugged.
+ - Plug in the device to power, and connect a HDMI display.
+ - Connect a micro-USB cable from the host PC to the target board.
+ - Power on the device by holding the start button until the red light is lit.
+ - Hold down the RST button and REC button simultaneously.
+ - Release the RST button while holding down the REC button.
+ - Wait a few seconds, then release the REC button.
+
+To flash over USB:
+
+```
+cd output/images/linux4tegra
+sudo bash ./flash.sh \
+     -I ../rootfs.ext2 \
+     -K ../Image \
+     -L ../u-boot-dtb.bin \
+     -C "ramdisk_size=100000 net.ifnames=0 elevator=deadline" \
+     -d ../tegra186-quill-p3310-1000-c03-00-base.dtb \
+     jetson-tx2-devkit mmcblk0p1
+```
+
+This will run the `flash.sh` script from L4T, and will setup the kernel, u-boot,
+persist + boot-up partition mmcblk0p1. This may overwrite your existing work so
+use it for initial setup only.
+
+Bootup Process
+==============
+
+The TX2 boots from the internal eMMC, at mmcblk0p1.
+
+A "secure boot" process is used, with multiple bootloaders:
+
+ - BootROM -> MB1 (TrustZone)
+ - MB2/BPMP -> (Non-Trustzone)
+ - Cboot (uses Little Kernel)
+ - Uboot
+ - Kernel
+
+Uboot is flashed to the mmcblk0p1 emmc partition.
+
+Cboot could be compiled from source, and the source is available from the
+official sources, however, we do not (yet) compile cboot.
+

board/jetsontx2

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+./jetson/tx2

configs/jetsontx2_defconfig

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+BR2_aarch64=y
+BR2_cortex_a57=y
+
+# enable specific optimizations
+BR2_TARGET_OPTIMIZATION="-march=armv8-a+crypto -mcpu=cortex-a57+crypto"
+
+# Toolchain reference: docs.nvidia.com: "Jetson Linux Driver Package Toolchain"
+BR2_TOOLCHAIN_BUILDROOT_CXX=y
+BR2_BINUTILS_VERSION_2_32_X=y
+
+BR2_SYSTEM_DHCP="eth0"
+
+# Linux headers same as kernel, a 4.9 series
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_9=y
+BR2_KERNEL_HEADERS_AS_KERNEL=y
+
+BR2_LINUX_KERNEL=y
+BR2_LINUX_KERNEL_CUSTOM_TARBALL=y
+BR2_LINUX_KERNEL_CUSTOM_TARBALL_LOCATION="$(call github,madisongh,linux-tegra-4.9,0be1a57448010ae60505acf4e2153638455cee7c)/linux-tegra-4.9.140-r1.tar.gz"
+BR2_LINUX_KERNEL_DEFCONFIG="tegra"
+
+# Build the DTB from the kernel sources
+BR2_LINUX_KERNEL_DTS_SUPPORT=y
+BR2_LINUX_KERNEL_INTREE_DTS_NAME="_ddot_/_ddot_/_ddot_/_ddot_/nvidia/platform/t18x/quill/kernel-dts/tegra186-quill-p3310-1000-c03-00-base"
+
+BR2_LINUX_KERNEL_NEEDS_HOST_OPENSSL=y
+
+BR2_PACKAGE_LINUX4TEGRA=y
+BR2_PACKAGE_LINUX4TEGRA_PLATFORM_T186REF=y
+
+# TODO: NVIDIA_CONTAINER_TOOLKIT requires a go-module integration.
+# BR2_PACKAGE_NVIDIA_CONTAINER_TOOLKIT=y
+
+BR2_PACKAGE_LINUX_FIRMWARE=y
+BR2_PACKAGE_LINUX_FIRMWARE_RTL_88XX=y
+
+# Required tools to create the image
+BR2_PACKAGE_HOST_DOSFSTOOLS=y
+BR2_PACKAGE_HOST_JQ=y
+BR2_PACKAGE_HOST_PARTED=y
+
+# Filesystem / image
+BR2_TARGET_ROOTFS_EXT2=y
+BR2_TARGET_ROOTFS_EXT2_4=y
+BR2_TARGET_ROOTFS_EXT2_SIZE="1024M"
+# BR2_TARGET_ROOTFS_TAR is not set
+
+# Uboot
+BR2_TARGET_UBOOT=y
+BR2_TARGET_UBOOT_BOARD_DEFCONFIG="p2771-0000-500"
+BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG=y
+BR2_TARGET_UBOOT_CUSTOM_TARBALL=y
+BR2_TARGET_UBOOT_CUSTOM_TARBALL_LOCATION="$(call github,paralin,u-boot-tegra,e6da093be3cc593ef4294e1922b3391ede9c94da)/u-boot-tegra-l4t-r32.4-v2016.7.tar.gz"
+BR2_TARGET_UBOOT_FORMAT_DTB_BIN=y
+BR2_TARGET_UBOOT_NEEDS_DTC=y