Good news!
I managed to make the USB-dongle compatible, paired it successfully and I managed to open up a port ( /dev/rfcomm1 ) between the Dongle and the pairing partner (HC-05)! And I managed to send "something" from the usb-bluetooth-dongle (@ YUN) to the HC-05 (@ UNO) ....
Currently I'm trying to send a "4" (digit) from the YUN to the UNO with these commands....
echo 4 > /dev/rfcomm1
echo '4' > /dev/rfcomm1
echo "4" > /dev/rfcomm1
echo 52 > /dev/rfcomm1 ## 52 is the ASCII-representation of the digit 4
echo "52" > /dev/rfcomm1
echo '52' > /dev/rfcomm1
echo '00110100' > /dev/rfcomm1
echo "00110100" > /dev/rfcomm1
printf '00110100' > /dev/rfcomm1
or
echo -e '4' > /dev/rfcomm1
...etc...
or
echo -n '4' > /dev/rfcomm1
...etc...
or
echo -en '4' > /dev/rfcomm1
...etc...
...but non of them works correctly... sadly all the UNO receives are 8 zeros. Here is my sketch for the UNO (currently there is NO sketch on the YUN, because its not necessary...all we need is the linino (for bluetooth) if we make this work 
:
#include <LiquidCrystal.h>
#include <SoftwareSerial.h>// import the serial library
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
SoftwareSerial mySerial(9, 10); // DTX Pin of HC-05 to Pin 9 (=SoftSerial RX) of UNO and DRX to Pin 10 (=SoftSerial TX)
//byte c; // << tried that too...
char c;
void setup() {
mySerial.begin(115200);
lcd.begin(16, 2);
}
void loop() {
lcd.clear();
delay(500);
if (mySerial.available() > 0) {
c = mySerial.read();
// DEBUGGING: TO SEE ON LCD WHAT WAS RECEIVED
lcd.setCursor(0, 1);
lcd.print(c);
delay(900);
}
}
My sketch works fine if two HC-05 communicate with eachother... but I don't know how to find out what the bluetooth-dongle is actually sending when I send the digit "4" with it...
EDIT:
Sending with the Dongle to the HC-05 is working fine now!
Now I just need to figure out how to receive with the dongle when the HC-05 sends something