Fix MOTD Release Notes, etc.

Resolves #135 (closed), and also includes items from !142 (closed), etc.

alpine.py:

• get release notes, rewrite to actual URL

build:

• --no-pad-uefi-bins skips padding of UEFI firmware binaries

clouds/aws.py:

• specify "gp3" volume type for EBS

configs/alpine.conf

• specify multiline MOTD sections as lists
• remove MOTD version_notes
• update MOTD release_notes

configs/arch/x86_64:

• allow UEFI firmware variant to also be built

image_configs.py:

• set release_notes from Alpine release information
• skip any MOTD release_notes if there are none
• skip any MOTD section without any value
• newline-join any MOTD section defined as a list

overlays/testing/configs/:

• add 'machine' dimension (VM vs Bare Metal)

support/aws/:

• add vmimport IAM role JSON files

Also add missing trailing newlines.