ARM: da850evm: Fix TI boot scripts for MMC

The da850evm include environment/ti/mmc.h and places
DEFAULT_MMC_TI_ARGS, defined int that file, in
CONFIG_EXTRA_ENV_SETTINGS.  This has been broken for some time
since neither CMD_PART nor CMD_UUID are available, so manually
changing the environmental variables was always required before
booting from MMC. With this patch, these scripts should work
again, and by default they point to mmc 0, partition 2, and
the dtb file exists in a /boot directory on partition 2.

Signed-off-by: Adam Ford <aford173@gmail.com>