Make explicit that libfdt should be built static

[crabbedhaloablution]

To avoid issues such as #333 , make explicit that libfdt should be built as static. This will allow raspberrypi-userland to more easily integrate with distributions which automatically add -DBUILD_SHARED_LIBS:BOOL=ON -DBUILD_STATIC_LIBS:BOOL=OFF to their CMake flags.

[HinTak]

I don't think it is advisable to guess what distributions need or do not need, unless you represent one of such distributions.

[crabbedhaloablution]

I'm working on a respin of the Gentoo ebuild. The default behavior of Gentoo matches that seen in #333 . The change is null for anyone else. I see no reason why it shouldn't be merged.