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

• mtools supports SOURCE_DATE_EPOCH. It can be verified with: mdir -i /path/to/efiboot.img :: -/a
• 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 (see #40).
• Add a reminder to not get rid of the dosfstools dependency, since a mformat made FAT image can fail to boot on some systems.