mvebu: base-files: Update Turris Omnia U-Boot environment

mvebu: base-files: Update Turris Omnia U-Boot environment

Move the update procedure from sysupgrade to first boot, which is much
more convenient in the sysupgrade case (otherwise the environment is
always one generation behind).

Check whether we have an old U-Boot release installed, and update the
environment only if necessary.

Some notes on the U-Boot environment:

The first 9 lines are a copy of the default environment of the old U-Boot
release - only modified, to run "distro_bootcmd", in case "mmcboot" fails
to boot the factory OS.

The remaining 16 lines are a backport of the default environment of the
new U-Boot release (shipped with CZ11NIC23). The main entry point is
"distro_bootcmd", which eventually sources boot.scr. This way, we have
a unified boot protocol for all Turris Omnia revisions so far.

This commit also fixes a shortcoming of previous Turris Omnia support:

Users may install OpenWrt with the Turris Omnia in factory state
(i.e. invalid environment store). In that case, neither fw_setenv, nor
U-Boot itself, would import the default environment from the image -
screwing up the rescue system, at least!

Signed-off-by: Klaus Kudielka <klaus.kudielka@gmail.com>
Reviewed-by: Tomasz Maciej Nowak <tomek_n@o2.pl>
Tested-by: W. Michael Petullo <mike@flyn.org> (Turris Omnia "2020")
Tested-by: Klaus Kudielka <klaus.kudielka@gmail.com> (Turris Omnia)