sabrelite HW setup: add serial monitoring for USB OTG

Mention the additional terminal for `minicom` for USB OTG boot
method. Need to mention this explicitly to avoid the problem
then novice user tries to run all steps in the single terminal.
This is preventing the data loss from the U-Boot output via serial
console and allows to see the flash status.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>