Enable placing the BSP outside the kernel tree

[nattgris]

Preferably used with a rt-kernel that allows building without a BSP, or
at least one that is be built with an in-tree BSP which is based on the
same platform as the external one.

Don't link against the trace library, because it cannot be built without
a BSP in-tree, and it depends on possibly mismatching settings from that
BSP.

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