mk_mmc.sh: uname_r is more useful this way

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
mk_mmc.sh: quiet wget

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
mk_mmc.sh: this works fine for dtbs

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
mk_mmc.sh: use smarter features of bootloader

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