MFC r266938:

Allow customization of the brand displayed in the boot menu.
If the user specifies in /boot/loader.conf:

    loader_brand="mycustom-brand"

Then "mycustom-brand" will be executed instead of "fbsd-logo".

Submitted by:    alfred
Obtained from:   FreeNAS

(cherry picked from commit ca2a1ac3055e8ef1e6ced68853090e970d4003bf)