6728161: Add SKIP_BOOT_CYCLE feature to create boot jdk and use it during build
Summary: Needed BOOT_JAR_JFLAGS. Fixed PREVIOUS_RELEASE_IMAGE.
Reviewed-by: tbell
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html> <head> <title>OpenJDK README</title> </head> <!-- ====================================================== --> <hr noshade="noshade" size="3"> <center> <h1>OpenJDK README</h1> </center> <!-- ------------------------------------------------------ --> <hr noshade="noshade" size="3"> <h2>TBD</h2> <h2>Building the OpenJDK</h2> <p> Refer to <a href="README-builds.html">the OpenJDK Build README</a> for build instructions. <!-- ====================================================== --> <hr noshade="noshade" size="3"> </body></html>