Revert previous revert of bootstd support, backport bootstd fallback patches and enable on RPi4

The RPi4 support upstream has migrated from the distroboot boot mechanism to bootstd. This move broke the fallback approach that we've been using in Apertis when U-Boot was upgraded. We'd previously reverted these changes as a temporary measure.

We've implemented an alternative fallback approach for the bootstd mechanism that's now in upstream. Backport this mechanism, revert the previous changes and enable the new mechanism for RPi4.

Signed-off-by: Martyn Welch martyn.welch@collabora.com