cmake: Explicit dependency for mraa and interfaces

A handful of modules do not require mraa.  Captured this in
    src/CMakeLists.txt - only add mraa dependency for targets which
    use:

        upm_module_init(mraa ... )
            or
        upm_mixed_module_init(... REQUIRES mraa)

    All sensors which use UPM interfaces (src/interfaces) now
    explicitly add the interfaces target:

        upm_module_init(interfaces ... )
            or
        upm_mixed_module_init(... REQUIRES interfaces)

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/interfaces/CMakeLists.txt

@@ -4,6 +4,9 @@ set (module_src ${libname}.cxx)
 
 upm_module_init()
 
+# Add a PUBLIC include directory to the CMAKE src dir
+target_include_directories (${libname} PUBLIC ${CMAKE_SOURCE_DIR}/src)
+
 # Don't add the hpp files with upm_module_init, this allows
 # them to be installed separately
 set (module_hpp iADC.hpp