docs/manual: update the linux tools section

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Romain Naour <romain.naour@openwide.fr>
[Thomas:
 - Adjust the Config.in example to show that we now need to "select
   BR2_PACKAGE_LINUX_TOOLS"
 - Adjust the .mk file example to use $(LINUX_DIR) instead of $(@D)]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>