Add logic to detect if the net/bsdec2-image-upload port needs
to be installed. [1]

For the cw-ec2-portinstall and ec2ami targets, touch the
.TARGET file after completion to prevent duplicate invocations.

Add cw-ec2-portinstall and ec2ami to CLEANFILES.

Submitted by: cperciva[1]
MFC after: 3 days
Sponsored by: The FreeBSD Foundation