Created a single header file (`CO_driver.h`) to declare the functions
of the drivers API. This forces drivers to use a common API.
Previously, each driver declared these functions in its specific
`CO_driver.h` file. This causes two issues:
1. while these functions are used in target-independent code,
the declarations could be target specific, leading to API/ABI
compatibility issues.
2. the declarations were duplicated for each driver.
The `CO_driver.h` files for each driver were renamed to
`CO_driver_target.h`. This file is included from `CO_driver.h`.
Added a new header for types: `CO_types.h`. This file currently
holds the definition of `CO_ReturnError_t` that was previously
duplicated for all drivers. Added a few enum values to support
the `neuberger-socketCAN` code.
Janez