Add provision for external board/application

[biii-tech]

Currently, wm-sdk only searches for board and application in hardcoded paths.
that is wm-sdk/board & wm-sdk/source

This structure has the following shortcomings

1. we can not have a different repo of our wire pass mesh application and had to put our board and application files in a defined structure.
2. wm-sdk can not act as a submodule of our project repositories.

We have made some changes and made these paths configurable.
now our project's structure is a bit leaned

• project
  -- board
  --- our board files
  -- source
  --- our source files
  -- wm-sdk as a submodule

We have also added a possibility to define application-specific custom make targets to do the post-processing.

you can review changes done here biiitech@45a24a6
It might be helpful for others