ec2_ami: boot_mode is missing "uefi-preferred" option

[ypismerov-tmx]

Summary

There are cases uefi-preferred has to be set up for AMI to be able to work with AWS metal instances (dedicated hosts).
It is currently missing. Only uefi and legacy-bios modes are supported by boot_mode parameter
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ami-boot.html lists of currently supported boot modes.

Issue Type

Feature Idea

Component Name

ec2_ami

Additional Information

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