pkg-generic: improve incorrectly used package detection

pkg-generic: improve incorrectly used package detection

Currently, the check that packages we build are indeed enabled is done
at the time a package is configured.

This can come quite late in the build process, and does not provide
direct knowledge of the real culprit for the incorrect dependency.

However, we can improve these two issues quite easily, albeit at the
expense of a very slightly more complicated make code.

First, the check can not be done at the time we define the package, i.e.
in the inner-generic-pacakge, because all its dependencies might have
not been parsed yet, so we can't yet know whether it is enabled or not
(because we can't match the package name of the dependency to its
Kconfig variable yet).

But then, we know we have all packages definitions after we scanned the
the bundled packages, kernel, bootloaders and toolchains, as well as the
br2-external tree (if any).

So, at this location, we iterate through the list of enabled packages,
and check that the packages they each depend on are indeed enabled.

This allows us to:
 1- do the check very early, before any build action,
 2- report on the exact offending package very easily.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>