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

## 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https://github.com/armbian/build/blob/main/config/boards/README.md is a bit outdated and should be revised.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](https://github.com/armbian/build/pull/6424#issuecomment-2028022124))

**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](https://github.com/armbian/build/blob/4232661c37c50aefd303a6ceee26e5fe72217740/config/boards/nanopi-r6c.csc#L14-L21) and another example to rename [ethernet interfaces](https://github.com/armbian/build/blob/4232661c37c50aefd303a6ceee26e5fe72217740/config/boards/nanopi-r6c.csc#L23-L31), like done in some board configs
- [ ] Add example how to add mainline U-Boot with function hook ([example](https://github.com/armbian/build/blob/4232661c37c50aefd303a6ceee26e5fe72217740/config/boards/nanopi-r6c.csc#L33-L53))
- [ ] Check for other deprecated options
- [ ] Sort lists alphabetically
- [ ] Remove `ext2` from `BOOTFS_TYPE` example