mediatek: mt7623: rework images for U7623-02 board

mediatek: mt7623: rework images for U7623-02 board

Users of older OpenWrt versions need sysupgrade using the *emmc.img.gz
file once which will upgrade U-Boot and switch to the new image layout.
Users of the vendor firmware need to first flash the legacy image to
then sunsequently carry out a full-flash upgrade.

Alternatively the board can also be flashed using MediaTek's
proprietary SP Flash Tool.

Configuration as well as persistent MAC address will be lost once at
this point and you will have to redo (or restore) all configuration
manually. To restore the previous persistent MAC address users may set
it manually using

fw_setenv ethaddr 00:11:22:33:44:55

For future upgrades once running OpenWrt past this commit, the usual
*sysupgrade.itb file can be used.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>