ARM: omap3: evm: Enable booting 'fitImage' with DEFAULT_FIT_TI_ARGS

This commit uses the existing DEFAULT_MMC_TI_ARGS and
DEFAULT_FIT_TI_ARGS defintions to replace the 'mmc*' environment
variables in the configuration. The check for the 'boot_fit' is handled
like the 'am335x_*' boards with 'CONFIG_BOOTCOMMAND'.

Signed-off-by: Derald D. Woods <woods.technical@gmail.com>