Allow the usage of non-standard JDKs for builds

java_home works for "standard" JDKs, but it is often desirable to have custom OpenJDK builds in
locations such as /opt.

This patch allows picking a correct $ALT_BOOTDIR when $JAVA_HOME is set, hence allowing building
OpenJDK with custom OpenJDK builds.
Merge pull request #4 from jponge/master

Seems good, merged