Make fdt default on arm and powerpc.
This now includes cross compiled targets, where before we tried to
make it host-based.
Also, move the lists of default yes and no options to a variable.
In general, only build tools should get this treatment in bsd.own.mk.
Also, the use of TARGET* in the bsd.*mk files is discouraged, but
necessary here due to the ordering of things in buildworld. We make
the native case work by testing MACHINE_ARCH after TARGET_ARCH.
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