Updates for the CMake rule file.

Updates for the CMake rule file.

1) Search explicitly for python2.7

In systems where python3 is default or in build cases where the user has
already searched for and found python3 in CMake, store the python3
executable and search for python2.7.

2) Generate nanopb core protobuf files with CMake

Generate python output files used in turn by the nanopb generator
script. This removes the requirement of manually calling 'make' in the
nanopb/generator/proto directory.

3) Use nanopb options file if it exists

Look for nanopb options file and use in protobuf source and header
generation if it exists. The options file must have the same name and
path as the proto file, excluding the extension.