uboot-imx

embestmx6boards: convert to use config_distro_bootcmd.h

Since the Riot & Mars boards are dev boards it's likely people will want to
run standard distros on them. So replace the current boot scripts with the
standard one from config_distro_bootcmd.h

Signed-off-by: Iain Paton <ipaton0@gmail.com>
Tested-by: Nikolay Dimitrov <picmaster@mail.bg>