package_ipk: Improve failure debug output

package_ipk: Improve failure debug output

Currently if the dpkg-deb command fails you see an error message like this:

ERROR: nss-3.27.1-r0 do_package_write_ipk: opkg-build execution failed
ERROR: nss-3.27.1-r0 do_package_write_ipk: Function failed: do_package_ipk

which is pretty much useless. If we use subprocess.check_output, we see a
traceback and then:

Exception: subprocess.CalledProcessError: Command '<cmd>' returned non-zero exit status 1

Subprocess output:
<output>

which is much easier to debug from.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>