setting build.id in about.mappings

http://www.eclipse.org/forums/index.php/t/216978/

https://bugs.eclipse.org/bugs/show_bug.cgi?id=309144

Pass -Dbuild.id= when you perform your site.p2 action

The Buckminster property expansion falls back to Eclipse variable
expansion so this should work:

build_id = ${env_var:BUILD_ID}

That timestamp will differ between projects. It finds the last
modification in the subtree and uses that so it's not suitable to use as
a build_id.