bctoolbox: disable rpath

By default, bctoolbox adds the rpath to the shared library. Prevent this
by setting `CMAKE_SKIP_RPATH` [1] to a true value.

[1] https://cmake.org/cmake/help/latest/variable/CMAKE_SKIP_RPATH.html



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>