Update to 5.10.20 kills USB boot on Pi4

[5ft24dave]

Tested on multiple Pi4's (4GB and 8GB) and with the 32 bit and 64 bit kernels, all fail to boot from USB after firmware update to 5.10.20, version 48570ba

[CaptainMidnight]

Please see #258, the test firmware worked for me, although things have moved on as you'll see if you follow that now closed issue and the EDID issue if that applies in your case.

[CaptainMidnight]

Probably a duplicate of issue #258 probably worth keeping an issue open until updated firmware commit is released to stop others reporting the same issue.

[popcornmix]

Fix for #258 (and quite likely this) is in rpi-update firmware.