tools: add pkg-diff script

Compare two list of packages and show the differences.
Expected syntax of file:
<package_name> <version>

Use:
diff-pkg old.packages new.packages

To generate the list of packages:
* Red Hat / CentOS: rpm -qa --queryformat '%{NAME} %{VERSION}'
* Debian / Ubuntu: dpkg -l | grep '^ii' | awk '{print $2 "\t" $3}'

Note: the package list generation is done by eDeploy in build/packages.