uboot-mediatek: build fully-featured U-Boot for MT7988 RFB

uboot-mediatek: build fully-featured U-Boot for MT7988 RFB

Select many potentially useful options for the MT7988 RFB U-Boot builds.
The resulting loader is intended as a development tool and intends to be
generic. It does *not* have a default bootcmd set, but allows to boot
pretty much everything, including EFI executables.

To install this U-Boot build to the eMMC:
  opkg install mmc-utils partx-utils
  mmc bootpart enable 1 1 /dev/mmcblk0
  echo 0 > /sys/block/mmcblk0boot0/force_ro
  dd if=*mediatek_mt7988a-rfb-nand-emmc-preloader.bin of=/dev/mmcblk0boot0
  dd if=*mediatek_mt7988a-rfb-nand-emmc-gpt.bin of=/dev/mmcblk0
  partx -a /dev/mmcblk0
  dd if=*mediatek_mt7988a-rfb-nand-emmc-bl31-uboot.fip of=/dev/mmcblk0p3

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 1d5778e18d9a6bef26aff3a8594306fc45e9be76)