build: allow TVH_BUILD to be overridden

diff --git a/support/apt-update b/support/apt-update
index 482a036318a655ec65d4dca8142ed0ee6584fbdc..c87b29be5c08304d02d36dbd4d4fe476c41c03ab 100755 (executable)
--- a/support/apt-update
+++ b/support/apt-update
@@ -22,7 +22,7 @@ DIR=$(cd $(dirname "$0"); pwd)
 TVH_ROOT=$(cd "$(dirname "$0")"/..; pwd)
 
 # Builds
-TVH_BUILD="
+[ -z "$TVH_BUILD"] && TVH_BUILD="
 precise:i386:amd64
 trusty:i386:amd64
 vivid:i386:amd64