How to customize LITB image from LSDK

This topic shows steps to customize LITB by using a different kernel image instead of the existing kernel image.

Browse to the FlexBuild installation directory.
Modify the kernel image in linux_arm64_LS.its.
```
$ vi configs/linux/linux_arm64_LS.its
```
Save the changes done in the file.

Generate LITB using flex-builder.

$ source setup.env
$ flex-builder -i mkitb -r <distro_type>:<distro_scale> -a <arch>

For example:

$ source setup.env
$ flex-builder -i mkitb -r ubuntu:main -a arm64

INSTRUCTION: mkitb
DISTRO TYPE: ubuntu
DISTRO SCALE: main
....
....
/home/flexbuild_lsdk2004/build/images/lsdk2004_ubuntu_main_LS_arm64.itb     [Done]

Note: To create .itb file directly from .its file, run this command: mkimage -f <xyz.its> <xyz.itb>

Connect to the board console via terminal and run following commands at U-boot to boot the board with customized LITB.

=> ping $serverip
Using e1000#0 device
host 192.168.3.1 is alive
=>
Using e1000#0 device
host 192.168.3.1 is alive
=> tftp 0xa0000000 lsdk2004_ubuntu_main_LS_arm64.itb
Using e1000#0 device
TFTP from server 192.168.3.1; our IP address is 192.168.3.49
Filename 'lsdk2004_ubuntu_main_LS_arm64.itb'.
Load address: 0xa0000000
Loading: #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         ####################################
         9.8 MiB/s
done
Bytes transferred = 683506200 (28bd7a18 hex)
=> bootm 0xa0000000#lx2160ardb
## Loading kernel from FIT Image at a0000000 ...
   Using 'lx2160ardb' configuration
   Trying 'kernel' kernel subimage
     Description:  ARM64 Kernel
     Created:      2021-02-03   6:01:29 UTC
     Type:         Kernel Image
     Compression:  gzip compressed
     Data Start:   0xa00000d0
     Data Size:    14086432 Bytes = 13.4 MiB

Check timestamp in boot log to ensure that the board is booted with the updated kernel image in the customized LITB.

Starting kernel ...

[    0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd083]
[    0.000000] Linux version 5.4.3 (test@Ubuntu-18) (gcc version 7.5.0 (Ubuntu/Linaro 7.5.0-3ubuntu1~18.04)) #1 SMP PREEMPT Wed Feb 3 00:04:09 IST 2021
[    0.000000] Machine model: NXP Layerscape LX2160ARDB
[    0.000000] earlycon: pl11 at MMIO32 0x00000000021c0000 (options '')
[    0.000000] printk: bootconsole [pl11] enabled
[    0.000000] efi: Getting EFI parameters from FDT: