RFR: JDK-8188136: jib configure requires --src-dir for out of tree builds

[Erik Joelsson]

This patch makes it possible to run "bash <path to>/jib.sh configure" 
from an out of tree directory, without specifying --src-dir to jib. The 
framework used for the Jib CLI automatically defines env variables that 
may provide defaults for each parameter. In this case I use the 
JIB_SRC_DIR env variable to provide a default for the src dir from the 
jib launcher script.

While in there I noticed that some relative paths were wrong after the 
consolidation and I fixed those too which also covers JDK-8188123.

Webrev: http://cr.openjdk.java.net/~erikj/8188136/webrev.01/
Bugs:
https://bugs.openjdk.java.net/browse/JDK-8188136
https://bugs.openjdk.java.net/browse/JDK-8188123

/Erik