Automatically set Alpine and Ubuntu version defaults in make

Prior to !3592 (merged), a developer would have to set up specific target Alpine and Ubuntu versions in environment before proceeding, or an error would result:

Missing required arguments. TARGET_FLAVOR and TARGET_FLAVOR_IMAGE_VERSION must be defined
make[1]: *** [out/helper-images/prebuilt-alpine-x86_64.tar] Error 1
make: *** [helper-dockerarchive-host] Error 2

Let's just set reasonable defaults if these values aren't set so developers can avoid this step.

Relates to #29305 (closed)