Use --fast by default in build_image when --jobs is enabled.

common.sh

Index: common.sh
diff --git a/common.sh b/common.sh
index 262895be69ba8f27a4025b4b97bbb41242d01739..446be805edd1e47f1703566e43ad703cec5791aa 100644
--- a/common.sh
+++ b/common.sh
@@ -325,14 +325,13 @@ function die {
 }
 
 # Retry an emerge command according to $FLAGS_retries
-# The $EMERGE_JOBS flags will only be added the first time the command is run
 function eretry () {
+  $* && return 0
   local i=
   for i in $(seq $FLAGS_retries); do
     echo Retrying $*
-    $* $EMERGE_JOBS && return 0
+    $* && return 0
   done
-  $* && return 0
   return 1
 }