OK thanks guys for your help to progress on this. Very constructive.
To answer @Daniel: Yes I bet the USART0 works well with Debian. But not with a normal use in Android. At least in my humble understanding.
To answer @Gwideman
1) I have no problem receiving to Arduino if sending data from DragonBoard so now I concentrate in OUT from Arduino in IN to DragonBoard. The software is a small loop in Arduino sending "Hello\n\r" and looping every seconds.
2) Placing (DragonBoard side) the ttyHS0 in "cat -v < /dev/ttyHS0" (meaning input mode) I receive any kind of string of characters + my Hello, (which is often truncated) ello or Hel or whatever preceding or following by non ASCII characters. The reception is also sporadic and do not follow the one second loop.
To do this I simply use ADB or also the Console in UART1.
I also wrote an app in java to receive data from the ttyHS0 and receive the same string of data. (garbage + my data)
3) I use my oscilloscope and can see the data on both side 1.8 and 5V just fine. Signal 96UART0_RX and PD1_TX.
4) If I wire my Arduino Output into a FTDI serial->USB directly to the Arduino output (which is parallel to the DragonBoard) into my PC I get all my data, I mean just "Hello". Of course I wire only the RX into the FTDI.
I think we may search and understand how is the UART0 initialized by Android and if we do not have a conflict with the GPIO (GPIO_1 BLSP) and UART0 on that pin connection. That is my position now unless someone has more idea or knowledge.
Thank you again, I will try to participate at the next 96 Open Hours but not sure for this time.