pkg-infra: allow dumping reverse dependencies of a package

pkg-infra: allow dumping reverse dependencies of a package

Finding the packages that select another one in a specific configuration
is not very trivial:

  - when optional, the dependency is not expressed in Kconfig

  - looking at the .mk files is not very nice.

Introduce a way to dump reverse dependencies of packages, i.e. the list
of packages that directly depend on that package. Like for direct
dependencies, we limit the list to the first-order reverse dependencies.

Document it in the main help; use the opportunity to also document
foo-show-depends.

Signed-off-by: "Yann E. MORIN" <[email protected]>
Cc: Arnout Vandecappelle <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>