MFC: r287013 (partial), r294616, r314097 (partial)

MFC: r287013 (partial), r294616, r314097 (partial)

- Allow overriding the FDT slicer with a custom slicer.
- Teach the flashmap code about SPI flash.
- Allow different slicers for different flash types to be registered
  with geom_flashmap(4) and teach it about MMC for slicing enhanced
  user data area partitions. The FDT slicer still is the default for
  CFI, NAND and SPI flash on FDT-enabled platforms.
- In addition to a device_t, also pass the name of the GEOM provider
  in question to the slicers as a single device may provide more than
  one provider.
- Build a geom_flashmap.ko.
- Use MODULE_VERSION() so other modules can depend on geom_flashmap(4).
- Remove redundant/superfluous GEOM routines that either do nothing
  or provide/just call default GEOM (slice) functionality.
- Trim/adjust includes

git-svn-id: svn://svn.freebsd.org/base/stable/10@318159 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f