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>