* Match meanings of 'version' and 'release' to how Alpine uses them
* Use optional 'revision' to denote any same-release AMI rebuild
* Include CPU 'arch' in naming/description (may also offer 'aarch64' AMIs someday)
* Upgrade build instance to use Amazon Linux 2 AMIs
* Use env vars to pass details to 'make_ami.sh' instead of via CLI parameters
* make_ami.sh
+ minimum version/release shouldn't be overrideable
+ update APK tools & Alpine keys
+ check build's release vs. installed /etc/alpine-release
* standardize on 'linux-virt' kernel
+ no longer need 'kernel_flavor' variable
+ always install 'aws-ena-driver' package
+ always enable ena_support
* switch to 'variables.yaml' for config
+ update build/convert script to stringify certain keys that may contain arrays
+ copy from 'variables.yaml-default' if it doesn't exist
* drop 'vpc' variable, using 'subnet' derives the proper VPC to use
* fix chrony.conf (all pool.ntp.org references are changed to 169.254.169.123)
* update README.md caveats