Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- There now exist options that have different defaults depending on
the architecture, reflect this in documentation. For such
options, both WITH_FOO and WITHOUT_FOO description files should
be provided.

Prodded by: des

- Setting a build option may enforce other build options, try harder
to detect this case.

- Setting a build option may change other option's default value,
try harder to detect this case.

Tidy up formatting.

The default for FDT has changed, rename the flag/options file.

Introduce a new build knob for Flattened Device Tree support.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation