Eliminate FDT being treated specially. When the BSDL DTC was
introduced, the meaning of WITH_FDT changed from 'do FDT in boot
loader and build dtc compiler' to be simply 'do FDT things in the boot
loader'. Now that we test for FDT in the architecture specific files,
this test here is now redunant and encourages bad behavior, so just
eliminate it and make it always yes. Those architectures whose boot
loaders don't support FDT already omit it when it is set to yes
anyway.