build-iso: update README build requirements for some packages

Added extra optional packages to the build requirements list in the
README file; required for building packages vyatta-cfg and
vyatta-cfg-vpn.

Bug #316 http://bugzilla.vyos.net/show_bug.cgi?id=316
Merge pull request #15 from cyclops8456/instructions

build-iso: update README build requirements for some packages
Update submodule build info.
Add vyos-vxlan submodule.
Add vyos-vxlan to packagelists.
Remove vxlan submodule, better just keep it in vyatta-cfg-system.
Remove vxlan from packagelists, better keep it in vyatta-cfg-system.
Merge branch 'instructions' into helium
Merge remote-tracking branch 'upstream/helium' into helium