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" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -974,6 +974,8 @@ help:
 	@echo '  <pkg>-depends          - Build <pkg>'\''s dependencies'
 	@echo '  <pkg>-configure        - Build <pkg> up to the configure step'
 	@echo '  <pkg>-build            - Build <pkg> up to the build step'
+	@echo '  <pkg>-show-depends     - List packages on which <pkg> depends'
+	@echo '  <pkg>-show-rdepends    - List packages which have <pkg> as a dependency'
 	@echo '  <pkg>-graph-depends    - Generate a graph of <pkg>'\''s dependencies'
 	@echo '  <pkg>-dirclean         - Remove <pkg> build directory'
 	@echo '  <pkg>-reconfigure      - Restart the build from the configure step'

package/pkg-generic.mk

@@ -698,6 +698,9 @@ $(1)-show-version:
 $(1)-show-depends:
 			@echo $$($(2)_FINAL_ALL_DEPENDENCIES)
 
+$(1)-show-rdepends:
+			@echo $$($(2)_RDEPENDENCIES)
+
 $(1)-graph-depends: graph-depends-requirements
 			@$$(INSTALL) -d $$(GRAPHS_DIR)
 			@cd "$$(CONFIG_DIR)"; \
@@ -854,6 +857,10 @@ $$(foreach pkg,$$($(2)_PROVIDES),\
 	$$(eval $$(call virt-provides-single,$$(pkg),$$(call UPPERCASE,$$(pkg)),$(1))$$(sep)))
 endif
 
+# Register package as a reverse-dependencies of all its dependencies
+$$(eval $$(foreach p,$$($(2)_FINAL_ALL_DEPENDENCIES),\
+	$$(call UPPERCASE,$$(p))_RDEPENDENCIES += $(1)$$(sep)))
+
 # Ensure unified variable name conventions between all packages Some
 # of the variables are used by more than one infrastructure; so,
 # rather than duplicating the checks in each infrastructure, we check