pkg-infra: add user-supplied step-hooks

Allow user to supply their own step-hooks by passing a variable
on the make command-line:
    make BR2_INSTRUMENTATION_SCRIPTS=/path/to/my/script

This can be useful to run site-specific actions at each step of the
build process, such as logging installed, removed or modified files,
do sanity checks on installed files...

It is possible to call more than one script, by passing a space-separated
lists of scripts to call.

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>