buildroot

core: add rule to dump packages' build order

When debugging hidden dependencies, the build order is very important.
Most notably, it is interesting to identify potential culprits.

Add a new top-level rule, show-biuld-order, that dumps all the packages
in the order they would get built.

Note that there are a few differences with show-targets:

  - more packages are reported, becasue show-targets does not report
    host packages that have no prompt;

  - the output is line-based, because we're using $(info $(1)); getting
    a single output line like show-targets would require we use an
    actual command, like printf '%s ' $(1); but that takes a lot of
    time, while $(info $(1)) is almost instantaneous (the time to parse
    the Makefiles);

  - rootfs targets are not reported.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>