UP Squared 6000: describe how to install an image on the eMMC storage

Our sanity-check-manual test is expected to be run on a system with its rootfs on an eMMC storage and not on a USB device, but the Apertis documentation only describe how to install the image on a USB device. This MR adds a note on how to install apertis on the eMMC storage of the board.

infrastructure/apertis-issues#388 (closed) tests/apertis-test-cases!562 (merged)

Signed-off-by: Dylan Aïssi dylan.aissi@collabora.com