.travis.yml: default the --disable-system build to --static

It's fairly common to build qemu-user binaries with --static linking
so the binary can be copied around without libraries. Enable --static
in the default qemu-user build to cover this.

There are other qemu-user builds that use dynamic linking so they
should catch any problems there.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

.travis.yml

@@ -80,7 +80,7 @@ script:
 matrix:
   include:
     - env:
-        - CONFIG="--disable-system"
+        - CONFIG="--disable-system --static"
 
 
     # we split the system builds as it takes a while to build them all