Create directories and copy files to the FAT image using mtools instead of mounting the file system

- mtools supports SOURCE_DATE_EPOCH.
- The image file is operated on directly instead of mounting the file system. This is a prerequisite to limit the commands that run with root privileges. Related to #40 .
- Add a reminder comment to not get rid of the dosfstools dependency, since a mformat made FAT image can fail to boot on some systems.