Multiple sketch support

[ddowling]

Summary

Allow CMakeLists.txt files to contain multiple build_sketch() targets.

This PR fixes #2282

A lot of Arduino library projects will contain multiple example applications that demonstrate the library functionality. It is also common practice on larger embedded applications to have a number of small test programs that test specific parts of the systems so these can be tested in isolation from the overall application code. This change means all of the examples and tests can be built together from the top level CMakeList.txt file.

There is a dependency error when .ino files are not in the parent directory of the project that is fixed with this pull request.