menu: don't bother going interactive with just one menu item

If there is only one menu item available, prompting user to enter
choice makes little sense and just causes unnecessary boot delay. This
change makes menu_get_choice return the only one item when there is no
other choices.

Signed-off-by: Leon Yu <leoyu@nvidia.com>
Cc: Tom Warren <twarren@nvidia.com>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Thierry Reding <treding@nvidia.com>