hikey: Use mtools to avoid sudo during build

Instead of using sudo/mount to build the FAT image, use commands from
the mtools package to copy files into the FAT image.  This eliminates
the requirement to use sudo during the build.  It also removes a small
sleep necessary to make umount work.

Signed-off-by: David Brown <david.brown@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>