Cmake relpath fix

- The move to using the protoc plugin rather than the 2-step
method has broken the relative path feature due to the differences
in path handling.
- Use CMAKE_CURRENT_BINARY_PATH instead of FIL_PATH_REL when calling
protoc with nanopb plugin to avoid having files placed in an incorrectly
nested directory

Bytes init value fix

Updates descriptor.proto to the latest.

Many options have been added to descriptor.proto since the last time it
was pulled in and nanopb will not work on .proto files using the new
options.

Addressed more permanently by https://github.com/nanopb/nanopb/pull/241
but that PR makes a series of larger changes to get there.

Add fixed count option

Fix CMake options file dependency

Restore dependency of generated nanopb source and header files on an
options file, if it exists in the same directory and has the name
filename without extension.

Add nanopb CMake variable NANOPB_DEPENDS which is a list of files to be
manually set as dependencies for the generated source and header files.
If these dependencies are options files, the corresponding directory is
passed as an options path.

Always return size for messages with OneOf fields

Use a maximum calculation if submessage sizes are unknown.

Allow the elements of that single non-packed repeated fixed count field
to be interleaved among other (non-packed repeated fixed count) fields.
Multiple non-packed repeated fixed count fields are allowed if the
elements are not interleaved.

An error is returned if the count of a repeated fixed count field is not
equal the expected count.

Use protoc plugin mode for source generation in CMake

This commit uses protoc using the nanopb plugin to generate source files
in a single step instead of two. A new variable 'NANOPB_OPTIONS' can be
used to pass nanopb options to protoc.

The directory of a generated proto file is always passed as a search
path for option files.

This fixes the calculation of max encoded size for proto files with
imports.

Explicitly handle all enum values in switch

Add nanopb_PROTOC_PATH variable to CMakeLists.txt