am335x_evm: Add more variables and switch to DT booting.

Make bootcmd run findfdt so that we know what dtb file to load.  Add a
loadfdt command to load this file in.  Make mmcboot pass in ${fdtaddr}
and make the mmc section of bootcmd run loadfdt.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Tom Rini <trini@ti.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>