buildroot

infra: Add PRE_*_HOOKS for every step

Add PRE_*_HOOKS to all the different steps through which a package may go.

This will help avoid using POST_*_HOOKS to do tasks that should be done
in the PRE_*_HOOKS of the next step.
Otherwise, when the user would do a make foo-re<step>, this would not do
what was really intented, the POST_*_HOOK of the preceding step not
being executed.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
[ThomasDS: rebase, add images hooks to manual]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>