fs/common: lock modification times in $TARGET_DIR

fs/common: lock modification times in $TARGET_DIR

Make sure all files in $TARGET_DIR have a defined modification time
before to generate filesystems.

This work was sponsored by `BA Robotic Systems'.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>