allow injection of CMake flags through environment variable for sel4-sys

[wucke13]

I.e. when compiling on a weird system, it should be possible to append CMake flags. This allows overriding flags already set, so downstream consumers can easily fix our mistakes and wrongful assumptions about the build host.

[moritz-meier]

Maybe not really necessary, could also be done with an external sel4 build. But also doesn't hurt to have it.