core/pkg-generic: allow step hooks to fail a step

Currently, we touch our stamp files before we call the step hooks.
This means a step hook can not properly fail a step, since the stamp
file exists even if the hook exits in error, thus a subsequent 'make'
would not try to redo that step.

Fix that by calling the hooks before we touch the stamp files.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Peter Seiderer <ps.report@gmx.net>
Cc: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>