Introduce fallback for general cross compilation

The current approach of forcing people to pass the fully qualified prefix
of their toolchain to the configure script is verbose and something of a
chore for people who use the same toolchain to target several devices.

This allows you to set a single toolchain for use with all Qt targets via:

qmake -set CROSS_COMPILE foo

You can still explicitly override this toolchain, as originally mandated,
with the configure time device-option arguments.

Change-Id: Ibd3d940bb08fa09499533f9c661557e337a8421a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

mkspecs/features/device_config.prf

@@ -22,6 +22,11 @@ isEmpty(DEVICE_PRI) {
 
 exists($$DEVICE_PRI):include($$DEVICE_PRI)
 
+isEmpty(CROSS_COMPILE) {
+    #this variable can be persisted via qmake -set CROSS_COMPILE /foo
+    CROSS_COMPILE = $$[CROSS_COMPILE]
+}
+
 unset(DEVICE_PRI)
 unset(DIR)