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>

content/reference_hardware/imx6q_sabrelite_setup.md

@@ -77,7 +77,9 @@ USB-OTG.
 In addition to the items outlined in the "Required items" section you will need
 a USB to microUSB cable.
 
-- Download and build the imx loader:
+- Monitor the serial console of the SABRE Lite using a dedicated terminal emulator.
+  Do not close this terminal to be able to detect if flashing process works as described.
+- In an additional terminal download and build the imx loader:
 
 <!-- end list -->
 
@@ -123,7 +125,8 @@ a USB to microUSB cable.
 
 - Board should boot U-Boot and program the flash using the image on the SD
   card.
-- This step is needed **ONLY** in case if you see the U-Boot prompt `=>`,
+- This step is needed **ONLY** in case if you see the U-Boot prompt `=>` on
+  terminal attached to serial console,
   usually it happens if you flash different U-Boot version.  You have to reset
   default environment and start the boot process:
 
@@ -134,7 +137,7 @@ a USB to microUSB cable.
     run bootcmd
 
 - Once the SABRE Lite has finished programming, the following message
-  will be displayed:
+  will be displayed on terminal attached to serial console:
 
 <!-- end list -->