ena(4) is not in GENERIC, now default for some/all instances on AWS EC2

[Maxim Sobolev]

Hi, today we've migrated one of our FreeBSD EC2 r4.xlarge instances and
painstakingly found that xn(4) interface is no longer provided by the
Amazon "hardware". ena(4) seems to be now default for a newly created VMs,
but it's not part of the GENERIC kernel. This could affect both new users
trying to deploy stock FreeBSD on AWS, as well as existing users migrating
their virtual assets running FreeBSD. I am not sure if there any technical
reasons for not having it, but perhaps somebody needs to take a look to add
it in there if there are none?

-Max