Add 'nrf5_bsp_defs' target with include dirs. only

[borys-jelenski-polidea]

There are examples in nRF5 SDK which rely on definitions found in bsp.h file however, they do not require the bsp.c to be compiled e.g. peripheral/uart. Actually, such examples do not contain the configuration required (in sdk_config.h) to compile the bsp.c. The problem is, when the nrf5_bsp target is linked in those examples, it cannot be built correctly.

The solution would be to create an interface target nrf5_bsp_defs containing only the include directory for the bsp.h file.