Secure-Element Preprocessor definition ifndef wrapping

[bdesterBE]

Hello,

I would like to open a pull request to simply wrap each define in src/peripherals/soft-se/se-identity.h with #ifndef tags. For example:

/*!
 * App/Join server IEEE EUI (big endian)
 */
#ifndef LORAWAN_JOIN_EUI
#define LORAWAN_JOIN_EUI
#endif

This would allow users to overwrite these values from application level CMakeLists.txt files without a warning being produced, and removes the need to modify LoRaMac-node directly with hard-coded values when it is being used as a submodule.

Thanks, Brandon