loader: create a generic vendor sub-menu place holder

Add a dummy vendor menu entry on the main welcome menu. Vendors can override
this in their local.lua file to create whatever sub-menu they need for their
products.

Also fix the adding a 'welcome' entry as well based on a suggestion from Kyle.
Silly option menu code also from Kyle. They seem to work for me, but any
transcription error is likely mine.

Reviewed by: kevans@ (the vendor stuff)

(cherry picked from commit e7ccd5b4187d15cd91697f1f4e12cf40e3ce86c3)