target/ssh.py: Add dump_target support

This adds the dump_target support when the ssh command fails with
a 'No route to host'. This is will provide additional data when a
Qemu target fails to respond during autobuilder testing. This does
not fix 14002 [0], but may help track down why qemu looses networking

[0] https://bugzilla.yoctoproject.org/show_bug.cgi?id=14002



Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>