HACK: Boot rootfs from sd card by default

* Since the emmc isn't programed trying to load the fs from the emmc causes
  boot failures/kernel panic.

* The current bootcmd is set to:
  bootcmd=run findfdt; run mmcboot;setenv mmcdev 1; setenv bootpart 1:2; \
  setenv mmcroot /dev/mmcblk0p2 rw; run mmcboot;

My guess is the env variables should be set so that sd card boot (dt,kernel,fs)
is the default and then fallback to emmc if it fails (no sd card detected)

The current bootcmd attempts to set mmcroot to the sd card rootfs but that code
doesn't run due to mmcboot being ran early on.

Signed-off-by: Franklin Cooper Jr. <fcooper@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>