Change default hostname to b2qt-linux-${MACHINE}

This is used by QtCreator to automatically define devices

Change-Id: I7ddb179b2e65d225196857accb371b9d17d36ad9
Reviewed-by: Samuli Piippo <samuli.piippo@digia.com>

conf/local.conf.sample

@@ -49,7 +49,7 @@ PARALLEL_MAKE = "-j 4"
 #
 # This sets the default machine to be qemux86 if no other machine is selected:
 MACHINE ??= "qemux86"
-MACHINE_HOSTNAME ?= "b2qt-${MACHINE}"
+MACHINE_HOSTNAME ?= "b2qt-linux-${MACHINE}"
 
 #
 # Where to place downloads