mk_mmc.sh: cleanup: drop video-musb

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
mk_mmc.sh: these are the same

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
mk_mmc.sh: boot_uenv_txt_template: so are these

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
mk_mmc.sh: cleanup: drop SYSTEM

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>