RIOT/Makefile.include: added indicating of possibe conflicting features on compile

Using `SPI0` and `DAC0` on the `stm32f4discovery` at the same time, may result in unexpected behaviour as the `DAC0` is internally wired with the `SPI0` port of the board.

With this PR the user is informed during compile time whenever multiple, probably conflicting, features set in the variable `EXPECT_CONFLICT` in the `Makefile.features` of the individual board, are also required by a project.
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