lualoader: use more concise verbiage for autoboot

The behavior remains the same, but lualoader now uses the more concise
verbiage that forthloader used. This is particularly important because
the previous line would exceed the right boundary of the menu and run
straight into space that would typically be allowed for the logo.

This makes it slightly easier to port logos from forthloader to
lualoader.

(cherry picked from commit ed107537b43cabf7a18e73a17856a9d9e170c6e9)