Add python-xdg to precise_cloud image

python-xdg is needed by the pre-built image job. To avoid apt-get failure, we need to include it in the custom AMI.

See <https://ci.linaro.org/jenkins/job/precise-armhf-pre-built-images-panda/25/>

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Started by timer
Building remotely on i-0918c972 in workspace <https://ci.linaro.org/jenkins/job/precise-armhf-pre-built-images-panda/ws/>
[precise-armhf-pre-built-images-panda] $ /bin/sh -xe /tmp/hudson4960943448855577198.sh
+ apt-get install -y bzr python-xdg
E: Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable)
E: Unable to lock the administration directory (/var/lib/dpkg/), is another process using it?
Build step 'Execute shell' marked build as failure