builder: use x-terminal-emulator

Don't hardcode matchbox-terminal, but use the alternative.

(From OE-Core rev: 3066d201633245ff53683ee12810977f25dde148)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-graphics/builder/files/builder_session.sh

@@ -30,4 +30,4 @@ fi
 cd /home/builder/poky
 . ./oe-init-build-env
 
-matchbox-terminal&
+x-terminal-emulator &