Changes to allow booting from eMMC on a Beaglebone Black. Using the
right config (am335x_boneblack_defconfig) is a big part of that. The
u-boot environment is also pretty extensively changed, both to support
eMMC (choosing which device to load freebsd from) and to further trim
and clean up the environment so it's easier to read and understand.

In an ideal world, the 'mmc 0' device would always be the one u-boot was
loaded from and 'mmc 1' would be 'the other' device. That's what the
Wandboard u-boot does, and it would be easy enough to do on Beaglebone too
except I couldn't find a way to pass the load device number from MLO which
knows it to the main u-boot. So for now the removable sdcard is device 0
and the onboard eMMC is device 1, and it prefers to boot from sdcard if
it's present and has ubldr on it (unless the user has set the 'fatdev' env
var, in which case it is used).

Approved by: imp