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

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

Fix problem where memcmp would return true in the case where the
character after the character that was the same.

Submitted by: jhay

MFp4: boot on KB9202 correctly. Also, reduce the size of the SD/MMC driver
somewhat.

MFp4:

Massive update. The highlights:
o dramatically cut memory usage by writing better, less intertwingled
code.
o implement booting off mmc/sd cards (sd only tested one at the moment)
o start to split out board specific stuff for boot2.