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

sys/boot/forth/brand.4th

@@ -56,6 +56,8 @@ variable brandY
 \ 	NAME        DESCRIPTION
 \ 	fbsd        FreeBSD logo
 \ 
+\ NOTE: Setting `loader_brand' to the value of an existing function
+\       (such as "mycustom-brand") will cause that symbol to be executed.
 \ NOTE: Setting `loader_brand' to an undefined value (such as "none") will
 \       prevent any brand from being drawn.
 \ 
@@ -87,5 +89,14 @@ variable brandY
 		2drop exit
 	then
 
+        \ if it refers to a raw symbol then run that function
+        sfind if
+            brandX @ brandY @
+            2 roll
+            execute
+        else            
+            drop
+        then
+
 	2drop
 ;