CMakeLists not set up properly for use as submodule.

[agribov]

There are a few problems with libuavcan CMakeLists.txt.

1. There is no call to target_include_directories, meaning that when the user links to uavcan the user application cannot reference the uavcan include folder without adding this themselves. Should add:

target_include_directories(uavcan
        PUBLIC
        ${CMAKE_CURRENT_SOURCE_DIR}/libuavcan/include
        ${CMAKE_CURRENT_SOURCE_DIR}/libuavcan/include/dsdlc_generated/
)

2. It always tries to build tests on debug builds. Should allow this to be turned on/off with a debug flag. Otherwise, this builds for all users who are including it as a submodule and making debug builds.