Improve unboxing experience

Some minor improvements make it maybe clearer what to do after unpacking;

• Looking at https://developer.puri.sm/Librem5/Development_Environment/Boards/imx8.html#test-run the "test run" step should go. Flashing the emmc should (in my opinion) be the very first thing - the board is pretty useless without an up to date image.
• It might make sense to point to the debs so people don't have to build by hand, setup udev rules, etc.
• Having a picture what the user sees after a succesfull flash and reboot might help
• fix the mention of using root/root: currently only works with UART until an upstream patch is accepted (MR)allowing root access to Debian using a USB serial connection
• Mention that in order to see the board under lsusb (right after unboxing), the boot mode must be usb (not eMMC)
• The build by hand section says cd mfgtools-build, then cmake ../mfgtools then make when `make needs to be run from inside the mfgtools dir (instead of mfgtools-build dir)