RFR(XS): 8054009: Support SKIP_BOOT_CYCLE=false when invoked from JPRT

[Mikael Vidstedt]

Please review the following small fix which re-introduces support for 
the SKIP_BOOT_CYCLE=false option when the build is invoked from JPRT. 
The support for doing the bootcycle build was accidentally lost when the 
change was made to have jprt run configure[1]. The fix simply adds a 
conditional dependency on the the bootcycle target when 
SKIP_BOOT_CYCLE=false is been specified.

Bug: https://bugs.openjdk.java.net/browse/JDK-8054009
Webrev (jdk9/dev): 
http://cr.openjdk.java.net/~mikael/webrevs/8054009/webrev.00/webrev/

This will also need to be backported to jdk8u/jdk8u-dev, so here's the 
"optimistic" backport:

Webrev (jdk8u/jdk8u-dev): 
http://cr.openjdk.java.net/~mikael/webrevs/8054009_8udev/webrev.00/webrev/

Cheers,
Mikael

[1] https://bugs.openjdk.java.net/browse/JDK-8029797