sys/conf: move libfdt path inclusion to kern.pre.mk

[ehem_freebsd_m5p.com]

Summary

As all architectures now include libfdt, it should be in the common
Makefile, rather than duplicated in every architecture.

Switch to per-library include lines, the better to make future
adjustments smaller and simpler.

[ehem_freebsd_m5p.com]

Updating in light of commit 8e53cd709943.

[imp]

sys/conf/Makefile.amd64

34

This line can be removed here and elsewhere.

[ehem_freebsd_m5p.com]

sys/conf/Makefile.amd64

34

True. I thought it better to leave the empty line behind to keep the Makefile.<arch> files more consistent with each other (in case a new include needs to be added). Perhaps leave as commented lines?

I don't have strong feelings on this though, feel free to simply nuke the lines on commit.