manual: mention LIBFOO_EXTRACT_CMDS

[Samuel: add precision about non-tar archive]
[Thomas: reword with more details. Fix commit title]
Signed-off-by: Stephan Hoffmann <sho@relinux.de>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>