build: Support a fallback to java 8 if java 7 is not found

 * In the case of a host installed only with java 8,
    the build starts correctly but on use of JAVA_HOME
    binaries, the java references are missing

 * Allow the java 7 to be tested and try to use java 8

Change-Id: Ie50892edb54feb8608a0ef80a1c2a8667703e8ee