loopback_test: add option to list loopback devices
loopback_test: add option to list loopback devices

Add the -l cmdline option so the user can see all known
loopback devices and their index to use in the mask

Reviewed-by: Bryan O'Donoghue <165dc05575478af93ac2f97f67be6e1756010094@linaro.org>
Signed-off-by: Axel Haslam <66bbaa7bd8fe83108c810be839e25f81f43cf794@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <4645f7897fd33786a2ee1264d590b3c400559d85@google.com>

projectara/gbsim,gregkh/gbsim,projectara/gbsim,gregkh/gbsim