Add /etc/autofs/special_efs to EC2 AMIs

Since Amazon Elastic File System is only available within AWS, it seems
more appropriate to have this added only in EC2 AMIs rather than
"polluting" non-EC2 images with it.

Reviewed by: gjb
MFC after: 7 days
Relnotes: Amazon EFS filesystems can be automounted by enabling autofs

		and placing "/efs -efs" into /etc/auto_master.

Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D24791