CB-4403 Combined check_reqs and init to allow prerequisite check before each script and a more simplified code flow

-Fixed error when check_reqs tried to run signing-utils from incorrect directory
-Updated all child calls in batch files [Windows] to exit on error.

Reviewed by Jeffrey Heifetz <jheifetz@blackberry.com>
Tested by Tracy Li <tli@blackberry.com>
CB-5237 Implemented config-file support in config.xml to inject custom xml into the bar-descriptor.xml

Reviewed by Bryan Higgins <bhiggins@blackberry.com>
Tested by Jenny Gee <jgee@blackberry.com>