View source for Jetson/Boot From External Device

== Introduction ==
* Flash device with rootfs on external devices.
* With new releases >= rel-32, external (USB, SATA, MMC) is being used as identification is solely on UUID based and not type of media.

== Steps ==
1. Create GPT and partiton using gdisk on external device:
   $sudo gdisk /dev/sd'''x'''
   "o" -> clear all current partition data
   "n" -> create new partition
   "1" -> partition number /dev/sd'''x'''1
   "40M"first sectors -> Press enter or "+32G" last sectors
   "Linux filsystem" -> using default type
   "c" -> partition's name "PARTLABEL"
   "w" -> write to disk and exit. 
2. Check partition UUID:
   $ sudo blkid /dev/sd'''x'''1
3. Copy PARTUUID to bootloader/l4t-rootfs-uuid.txt

4. Copy rootfs to SDcard:
   $ sudo mkfs.ext4 /dev/sd'''x'''1  (check PARTUUID again)
   $ sudo mount /dev/sd'''x'''1 /mnt
   $ cd rootfs/
   $ sudo tar -cpf - * | ( cd /mnt/ ; sudo tar -xpf - )
   $ sudo umount /mnt
5. Flash the board with rootfs device (external is for sdcard, usb, sata and mmc):
   $ sudo ./flash jetson-tx2 external

== Check ==
After boot up, the u-boot logs will like below:
 TEGRA186
 Model: NVIDIA P2771-0000-500
 DRAM:  7.8 GiB
 MC:   Tegra SD/MMC: 0, Tegra SD/MMC: 1
 *** Warning - bad CRC, using default environment
 In:    serial
 Out:   serial
 Err:   serial
 Net:   eth0: ethernet@2490000
 Hit any key to stop autoboot:  0 
 switch to partitions #0, OK
 mmc1 is current device
 Scanning mmc 1:1...
 Found /boot/extlinux/extlinux.conf
 Retrieving file: /boot/extlinux/extlinux.conf
 733 bytes read in 127 ms (4.9 KiB/s)
 L4T boot options
 1:	primary kernel
 Enter choice: 1:	primary kernel
 Retrieving file: /boot/initrd
 5564673 bytes read in 697 ms (7.6 MiB/s)
 Retrieving file: /boot/Image
 34179080 bytes read in 3848 ms (8.5 MiB/s)
 append: '''root=PARTUUID=ec27c846-88af-41f6-bd4b-fd46c54bb5ce''' rw rootwait rootfstype=ext4 console=ttyS0,115200n8 console=tty0 fbcon=map:0 net.ifnames=0 
 video=tegrafb no_console_suspend=1 earlycon=uart8250,mmio32,0x3100000 nvdumper_reserved=0x2772e0000 gpt tegra_fbmem2=0x800000@0x9607b000 
 lut_mem2=0x2008@0x96078000 usbcore.old_scheme_first=1 tegraid=18.1.2.0.0 maxcpus=6 boot.slot_suffix= boot.ratchetvalues=0.2031647.1 
 bl_prof_dataptr=0x10000@0x275840000 sdhci_tegra.en_boot_part_access=1 quiet