[cloud] Allow multiple images to be imported simultaneously

[cloud] Allow multiple images to be imported simultaneously

Allow both x86_64 and arm64 images to be imported in a single import
command, thereby allowing for e.g.

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

  make CONFIG=cloud EMBED=config/cloud/aws.ipxe \
       CROSS=aarch64-linux-gnu- bin-arm64-efi/ipxe.usb

  ../contrib/cloud/aws-import -w amilist.txt -p \
       bin/ipxe.usb bin-arm64-efi/ipxe.usb

This simplifies the process of generating a single amilist.txt file
for inclusion in the documentation at https://ipxe.org/howto/ec2

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