pkg-infra: introduce pre/post-step hooks

This hooks will let us instrument the build process in many ways:
  - log current step to see what broke
  - time each step to see what is worth optimising
  - sanity-check installed files (rpath, overwritten files...)
  - call user-provided script
  - ...

The steps are coarse-grain, and all have a 'start' and a 'end' hooks.
Here is the list of available steps (8 total):
  - extract
  - patch
  - configure
  - build
  - install-host
  - install-staging
  - install-image
  - install-target

The download, clean and uninstall steps are not instrumented on purpose.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>