env: ti: boot: Extract command for eMMC Linux boot

Extract commands for booting Linux from eMMC to separate command. It
seems more logical that way, and allows us to run the whole command set
from U-Boot shell with only one command.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>