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

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

MFp4: formatting nits and minor style changes that likely won't matter.

MFp4 (checkpoint of work in progress):

o Use a directory layout that is more akin to the i386 boot layout.
o Create a libat91 for library routines that are used by one or more
of the boot loaders.
o Create bootiic for booting from an iic part.
o Create bootspi for booting from an spi part.
o Optimize the size of many of these routines (especially emac.c). Except
for the emac.c optimizations, all these have been tested.
o eliminate the inc directory, libat91 superceeds it.
o Move linker.cfg up a layer to allow it to be shared.

Need at91rm9200_lowlevel.h for SDRAM_BASE definition.

Recovery boot loader for the AT91 family of processors. Download it
via xmodem to the DBGU port when the AT91 comes up in recovery mode.
The recovery loader will then load your program via xmodem into SDRAM
at 1MB which can do its things. It needs to be tweaked to the
specific board one is using, but it fits in < 1kB (all of Atmel's ARM
products have at least 8kb of SRAM that I can tell, so this should
work for them all).

Parts of this code were provided by Kwikbyte with copyright
specifically disclaimed. I heavily modified it to act as a recovery
loader (before it was a bootstrap loader) and to optimize for size
(before I started the size was closer to 8k).

Bootstrap loaders for SPI and IIC to follow.