core/pkg-generic: add support for package-defined help

Add a package-variable to store the package-specific make rules.

Although this variable would be seldom used, we still document it.
However, we make sure the documentation explicitly states that this
variable should not be used (if it needs to be, the submitter of a
package will be told so during reviews).

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

@@ -937,6 +937,7 @@ help:
 	@echo '  <pkg>-dirclean         - Remove <pkg> build directory'
 	@echo '  <pkg>-reconfigure      - Restart the build from the configure step'
 	@echo '  <pkg>-rebuild          - Restart the build from the build step'
+	$(foreach p,$(HELP_PACKAGES),$($(p)_HELP_CMDS)$(sep))
 ifeq ($(BR2_PACKAGE_BUSYBOX),y)
 	@echo '  busybox-menuconfig     - Run BusyBox menuconfig'
 endif

docs/manual/adding-packages-generic.txt

@@ -497,6 +497,12 @@ different steps of the build process.
   systemd is selected as the init system in the configuration, only
   +LIBFOO_INSTALL_INIT_SYSTEMD+ will be run).
 
+* +LIBFOO_HELP_CMDS+ lists the actions to print the package help, which
+  is included to the main +make help+ output. These commands can print
+  anything in any format.
+  This is seldom used, as packages rarely have custom rules. *Do not use
+  this variable*, unless you really know that you need to print help.
+
 The preferred way to define these variables is:
 
 ----------------------

package/pkg-generic.mk

@@ -914,6 +914,10 @@ ifeq ($$(patsubst %/,ERROR,$$($(2)_SITE)),ERROR)
 $$(error $(2)_SITE ($$($(2)_SITE)) cannot have a trailing slash)
 endif
 
+ifneq ($$($(2)_HELP_CMDS),)
+HELP_PACKAGES += $(2)
+endif
+
 endif # $(2)_KCONFIG_VAR
 endef # inner-generic-package