uninative: show errors if installing fails

uninative: show errors if installing fails

We call the installer script but if it fails the user doesn't see the
reason, which makes resolving the problem hard.  Capture both stdout and
stderr as text, and display it to the user when it fails.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>