radxa-nio-12l: build u-boot from sources by setting BOOTCONFIG & BOOT_SOC

- BOOTCONFIG `genio_1200_radxa_nio_12l_d8_defconfig`
  - probably should be `d4` by default, but configurable
  - d4/d8/d16 defconfigs only affect how much ram u-boot sees, not the kernel
- BOOT_SOC `mt8195`
- those effectively enable u-boot build from source
  - incl optee, tf-a, fip
    - provided by family code

Author: rpardini

config/boards/radxa-nio-12l.conf

@@ -9,3 +9,5 @@ BOOT_FDT_FILE="mediatek/mt8395-radxa-nio-12l.dtb"
 SERIALCON="ttyS0:921600"
 HAS_VIDEO_OUTPUT="yes"
 declare -g BOARD_FIRMWARE_INSTALL="-full"
+declare -g BOOTCONFIG="genio_1200_radxa_nio_12l_d8_defconfig"
+declare -g BOOT_SOC="mt8195"