default empty tapas to TARGET_BUILD_APPS "all"

So that "tapas" without argument won't end in makefile-could-not-load error in unbundled branch.
Also, the magic "all" apps will build only apps, not other things.

Change-Id: Iac6dd928ee4e42996147acdab31930cd4ffa1768

envsetup.sh

@@ -546,6 +546,9 @@ function tapas()
     if [ -z "$variant" ]; then
         variant=eng
     fi
+    if [ -z "$apps" ]; then
+        apps=all
+    fi
 
     export TARGET_PRODUCT=generic
     export TARGET_BUILD_VARIANT=$variant