[611][512] Detect CM4 Lite variant using a mailbox tag
Previously we commandeered GPIO extender IO4 to be told whether eMMC is fitted or not, and only if we're certain it is via opt-in (the line being actively pulled low) would SDFS treat that as a fixed disc. Unfortunately, extender IO4 is used to set 1.8V versus 3.3V signalling on the SDIO bus - so some brands of SD card would not work as SD_VDD_override was pulled high to 1.8V signalling when 3.3V was needed.
- Use the 2 word form of ARM2VC_Tag_GetBoardRevision to deduce whether the CM4 is a Lite or a not-Lite
- If the firmware predates the 2 word form default to assuming SDIO media is removable so FileCore does the extra safety checks
- Set the internal antenna switch on CM4 so the selector isn't left in an undefined state