List the useful hook points available in buildroot

The complete list can be obtained with the following command:
  grep "_HOOKS[ ]\+?=" package/pkg-generic.mk

Signed-off-by: Davide Viti <zinosat@tiscali.it>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>