[Task]: Update board readme doc (`build/config/boards/README.md`)

[ColorfulRhino]

Task description

https://github.com/armbian/build/blob/main/config/boards/README.md is a bit outdated and should be revised.

Task list

Add to the list:

• [ ] BOOTBRANCH
• [ ] BOOT_SCENARIO
• [ ] BOOT_SUPPORT_SPI
• [ ] BOOT_SOC
• [ ] UBOOT_TARGET_MAP (good explanation here)

Add to deprecated:

• [ ] BOOTBRANCH_BOARD

Misc:

• [ ] Explain more about KERNEL_TEST_TARGET, what are tests even? Where and how are tests executed?
• [ ] BOOT_FDT_FILE example: add: Replace device tree with the one specified in the board's defconfig from U-Boot
• [ ] Add new section to explain hook functions in board configs
  • [ ] Add example to rename audios and another example to rename ethernet interfaces, like done in some board configs
  • [ ] Add example how to add mainline U-Boot with function hook (example)
• [ ] Check for other deprecated options
• [ ] Sort lists alphabetically
• [ ] Remove ext2 from BOOTFS_TYPE example

[github-actions[bot]]

Jira ticket: AR-2411