Enable fast build by default in build_image and mod_image_for_test.sh.

The fast build is stable enough now that we can enable it for everybody.
I ran the build constantly all weekend on two machines and I only ran into
one (rare) error, which I've fixed in a separate patch.

See <http://codereview.chromium.org/2856048/show>

TEST=Ran lots and lots of fast builds
BUG=none

Review URL: http://codereview.chromium.org/3059001

build_image

@@ -59,8 +59,8 @@ DEFINE_integer statefulfs_size 1024 \
 DEFINE_boolean preserve ${FLAGS_FALSE} \
   "Attempt to preserve the previous build image if one can be found (unstable, \
 kernel/firmware not updated)"
-DEFINE_boolean fast ${FLAGS_FALSE} \
-  "Call many emerges in parallel (unstable)"
+DEFINE_boolean fast ${DEFAULT_FAST} \
+  "Call many emerges in parallel"
 
 DEFINE_string usb_disk /dev/sdb3 \
   "Path syslinux should use to do a usb boot. Default: /dev/sdb3"

common.sh

@@ -105,6 +105,12 @@ ALL_BOARDS=$(echo $ALL_BOARDS)
 #DEFAULT_BOARD=x86-generic # or...
 DEFAULT_BOARD=$(echo $ALL_BOARDS | awk '{print $NF}')
 
+# Enable --fast by default on non-official builds
+DEFAULT_FAST="${FLAGS_TRUE}"
+if [ "${CHROMEOS_OFFICIAL:-0}" = "1" ]; then
+  DEFAULT_FAST="${FLAGS_FALSE}"
+fi
+
 # Detect whether we're inside a chroot or not
 if [ -e /etc/debian_chroot ]
 then

mod_image_for_test.sh

@@ -32,7 +32,7 @@ DEFINE_string qualdb "" "Location of qualified component file" d
 DEFINE_boolean yes $FLAGS_FALSE "Answer yes to all prompts" y
 DEFINE_string build_root "/build" \
     "The root location for board sysroots."
-DEFINE_boolean fast ${FLAGS_FALSE} "Call many emerges in parallel"
+DEFINE_boolean fast ${DEFAULT_FAST} "Call many emerges in parallel"
 
 
 # Parse command line