kas/: pass through DISPLAY from environment

If a user does eg "kas shell kas/corstone1000-base.yml" and then calls
runfvp, the spawned xterms don't have a valid DISPLAY set.

Add DISPLAY to the preserved environment variables and DISPLAY will be
passed into the shell, and the xterms will start correctly.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

kas/corstone1000-base.yml

@@ -1,6 +1,9 @@
 header:
   version: 11
 
+env:
+  DISPLAY: ""
+
 distro: poky-tiny
 
 defaults:

kas/corstone500.yml

@@ -3,6 +3,9 @@ header:
   includes:
     - kas/fvp-eula.yml
 
+env:
+  DISPLAY: ""
+
 distro: poky-tiny
 
 defaults:

kas/fvp-baser-aemv8r64-bsp.yml

@@ -3,6 +3,9 @@ header:
   includes:
     - kas/fvp-eula.yml
 
+env:
+  DISPLAY: ""
+
 distro: poky
 machine: fvp-baser-aemv8r64