[cloud] Add utility for importing images to AWS EC2

[cloud] Add utility for importing images to AWS EC2

Add a utility that can be used to upload an iPXE disk image to AWS EC2
as an Amazon Machine Image (AMI).  For example:

  make CONFIG=cloud EMBED=config/cloud/aws.ipxe bin/ipxe.usb

  ../contrib/cloud/aws-import -p -n "iPXE 1.21.1" bin/ipxe.usb

Uploads are performed in parallel across all regions, and use the EBS
direct APIs to avoid the need to store temporary files in S3 or to run
VM import tasks.

Signed-off-by: Michael Brown <mcb30@ipxe.org>