rockchip: only include libfdt in non-coreboot cases

While mainline u-boot always expects to submit the devicetree
as platform param, coreboot always uses the existing parameter
structure. As libfdt is somewhat big, it makes sense to limit
its inclusion to where necessary and thus only to non-coreboot
builds.

libfdt itself will get build in all cases, but only the non-
coreboot build will actually reference and thus include it.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Change-Id: I4c5bc28405a14e6070917e48a526bfe77bab2fb7