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

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

- Give geom_redboot taste of flash/spi. Now there is another provider
of redboot partitions. This patch was missed during merge from
projects/mips.

Do not stop the loop when an empty or deleted directory entry is found.
Rather just skip over it.

o disallow write to RedBoot and FIS directory partitions; these are painful
to resurrect (maybe honor foot shooting bit in kern.geom_debugflags)
o fix match macro so we now recognize we want to merge FIS dir with RedBoot
config parameters even if we don't actually do it

add geom_redboot, a geom module that exports RedBoot FIS partitions as named
slices in dev/redboot/*