Slim down the first run procedure

Epic: https://source.puri.sm/Librem5/use-cases/issues/62

On https://developer.puri.sm/Librem5/Development_Environment/Boards/imx8.html , the test run is sliced down the middle by the Software Upgrade note, which is somwehat confusing in two ways: while doing the test run, it's too early for the user to think about upgrading, and also the connection between

There are two simple ways to verify that the development board is functioning. It is useful to try both of these to ensure that you can access the board via serial and Ethernet connections.

and the two following section is lost.

Also, the wording of that paragraph is not very clear: the methods are only referenced implicitly in a subordinate clause.