[EEPROM][FDT_OVERLAY][ENV] Add EEPROM Manager, seco_fdt_overlay
Mostly cherry-picked commits from u-boot 2021.
Added:
- Use seco_fdt_overlay globally if the `CONFIG_CMD_SECO_FDT_OVERLAY´ config is set for board, otherwise the default one
- Added reading the second MAC Address from MTK repository
- Added autoload of custom overlays from u-boot 2021
Question to all: Can we maybe squash some commits to make the whole thing more portable?