VMware OEM and open-vm-tools

Based in part on https://github.com/coreos/coreos-overlay/pull/898 but with additional cross-compiling fixes and moved to open-vm-tools 9.4.6 instead of an old development snapshot.

This is still a work in progress but I think it is time to merge so it is easier for us to collaborate on this.

Remaining concerns/questions:
 - scripts installed to /etc are completely thrown away right now, what impact does this have? What breaks when there isn't a network kicking script? Also:  vmware/open-vm-tools#3
 - this is linking against glib and procps in the base CoreOS system, I am unsure how safe this is since the OEM partition does not get upgraded with /usr but it is ok for testing/development.

Thanks to @c4milo and @crawford for helping get this into reasonable shape.

Not fully tested yet...
The other variables are handled by the `AC_VMW_CHECK_LIB` macro defined in `m4/vmtools.m4`