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

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

MFp4: boot2 should now build

Add boot0iic and boot0spi. These are 'shortcut' boot loaders designed
to replace either a bad iic eeprom with one you download with xmodem,
or likewise with a spi flash.

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.