mediatek: add build for MT7981 RFB

Add build for the MTK3943 reference board for MT7981B+MT7976C.

**Hardware specification:**

 - SoC: MediaTek MT7981B 2x A53
 - Flash: various options
 - RAM: 256MB DDR3
 - Ethernet: 4 x 10/100/1000 Mbps via MT7531AE switch
        EITHER 1 x 10/100/1000 Mbps built-in PHY
            OR 1 x 10/100/1000/2500 Mbps MaxLinear GPY211C
 - Switch: MediaTek MT7531AE
 - WiFi: MediaTek MT7976C
 - Button: RST, WPS

**Flash instructions for SPIM-NAND:**
 - write *mt7981-rfb-spim-nand-preloader.bin to 'BL2' partition
 - write *mt7981-rfb-spim-nand-bl31-uboot.fip to 'FIP' partition
 - erase 'ubi' partition
 - reset board
 - create ubootenv and ubootenv2 UBI volumes in U-Boot
 - edit environment and set bootcmd, e.g.
   setenv bootconf 'config-1#mt7981-rfb-spim-nand#mt7981-rfb-mxl-2p5g-phy-eth1'
   setenv bootcmd 'ubi read $loadaddr fit; bootm $loadaddr#$bootconf'
 - load initramfs image via TFTP:
   setenv serverip 192.168.1.254
   setenv ipaddr 192.168.1.1
   setenv bootfile openwrt-mediatek-filogic-mediatek_mt7981-rfb-initramfs.itb
   saveenv ; saveenv
   tftpboot
   bootm $loadaddr#$bootconf
 - Now use sysupgrade to write OpenWrt firmware to flash.

SNFI-NAND, SPIM-NOR and eMMC all work very similar, a bootable SD card image
is also being generated. However, as the board I've been provided only comes
with SPIM-NAND all other boot media are untested.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit ce7209bd21661e3daa4a7f2f58dafdff990da19f)

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