Provide a command-line switch to set root-directory to begin b2-generation

[u3r]

Problem

If the b2-enabled project is not in the root-folder of the repository one has to change directory to build the project. This creates Problems in jenkins Builds, where a specific .pom (in a subdirectory) is specified.
B2 starts, detects no b2-project in current (root) directory, does not generate pom.xmls and maven fails with pom.xml not found.

Proposals:

1) reuse mavens own -f

This would solve simple cases auto-magically but may be too restrictive for complex build-setups

2) introduce own command-line switch