catalyst-auto: run update_symlinks by default in pre_build

All configs defining update_symlinks run it in pre_build, and all the
other configs will want to do the same.  So make it the default in the
pre_build phase.

tools/catalyst-auto

@@ -94,7 +94,7 @@ update_symlinks() {
 pre_build() {
   # This is a skeleton function that you can override from the config file.
   # It will be executed before the build is started.
-  :
+  update_symlinks
 }
 
 post_build() {

tools/catalyst-auto-amd64-experimental.conf

@@ -71,10 +71,6 @@ update_symlinks() {
 	done
 }
 
-pre_build() {
-	update_symlinks
-}
-
 post_build() {
 	cmd=(
 		rsync

tools/catalyst-auto-amd64.conf

@@ -75,10 +75,6 @@ update_symlinks() {
 	done
 }
 
-pre_build() {
-	update_symlinks
-}
-
 post_build() {
 	cmd=(
 		rsync