docs/manual: add documentation for linux-ext and linux-tools.

[Thomas: misc improvements.]

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>