Add backports to the chroot builder sources

This allows to have newer software than what is available in Debian Squeeze
(the default version of the distribution used by the script), particularly
vim and git.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

devel/build_chroot

@@ -62,13 +62,23 @@ debootstrap --arch $ARCH $DIST_RELEASE $CHDIR
 
 APT_INSTALL="apt-get install -y --no-install-recommends"
 
+echo "deb http://backports.debian.org/debian-backports" \
+     "$DIST_RELEASE-backports main contrib non-free" \
+     > $CHDIR/etc/apt/sources.list.d/backports.list
+
 #Install all the packages
 in_chroot -- \
   apt-get update
 
+#Install selected packages from backports
+in_chroot -- \
+  apt-get -y --no-install-recommends -t squeeze-backports install \
+    git \
+    vim
+
 in_chroot -- \
-  $APT_INSTALL python-setuptools build-essential python-dev sudo automake git \
-               fakeroot vim rsync locales
+  $APT_INSTALL python-setuptools build-essential python-dev sudo automake \
+               fakeroot rsync locales
 
 echo "en_US.UTF-8 UTF-8" >> $CHDIR/etc/locale.gen