Fix and improve Spring Boot rule debugging

[plaird]

First there is a bug in the debug log lines of the springboot packaging bash script:

The env variable name got lowercased in the script by mistake. Since it is a shell variable it should be put back to all caps, as the documentation describes.

Second, we should make the debug log lines in the springboot.bzl an option via an attribute. Create a new attribute debug that is default False.

[plaird]

Fixed the first issue - the script detects either lowercase or uppercase options for the foreseeable future. Will remove lowercase support later.

I now disagree with myself for the second. It is nicer for the debug flag to be an env variable, to easily allowing setting and unsetting it without modifying a BUILD file.