pkg-infra: introduce errors for legacy API

As discussed in the BR developer days, we want to be more strict about API
changes in buildroot. I.e., we want to make it less likely that a user's
customizations break down after upgrading buildroot.

A first step is to make sure that the user is warned about API changes.
This patch introduces Makefile.legacy and Config.in.legacy, which will
issue clear error messages for such situations.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Config.in

@@ -434,3 +434,5 @@ source "fs/Config.in"
 source "boot/Config.in"
 
 source "linux/Config.in"
+
+source "Config.in.legacy"

Config.in.legacy

@@ -0,0 +1,51 @@
+#
+# Config.in.legacy - support for backward compatibility
+#
+# When an existing Config.in symbol is removed, it should be added again in this
+# file, and take appropriate action to approximate backward compatibility. If
+# there is an equivalent (set of) new symbols, these can just be select'ed by
+# the old symbol. This makes sure that running 'make oldconfig' will make things
+# "just work" when upgrading to a new buildroot version. If the change is too
+# fundamental and cannot be fixed by a simple select, then the old symbol should
+# select BR2_LEGACY.  If that symbol is set, the build will issue an error.
+#
+# When adding legacy symbols to this file, add them to the front. The oldest
+# symbols will be removed again after about two years.
+#
+# The symbol should be copied as-is from the place where it was previously
+# defined, but the help text should be removed or replaced with something that
+# explains how to fix it.
+
+config BR2_LEGACY
+	bool
+	help
+	  This option is selected automatically when your old .config uses an
+	  option that no longer exists in current buildroot. In that case, the
+	  build will fail. Look for config options which are selected in the
+	  menu below: they no longer exist and should be replaced by something
+	  else.
+
+# This comment fits exactly in a 80-column display
+comment "Legacy detected: check the content of the menu below"
+	depends on BR2_LEGACY
+
+# This option should get a new name with every buildroot release, so it defaults
+# to y again for people who upgrade.
+menuconfig BR2_LEGACY_CHECK_2012_11
+	bool "Check for legacy config options"
+	default y
+	help
+	  Select this option to see the config options that are present in your
+	  current .config but are no longer supported by buildroot. If any of
+	  the options in this menu is selected, they should be replaced with
+	  something else. As long as they stay selected, the build will fail.
+	  Just de-select this option to automatically remove all the legacy
+	  configuration.
+
+if BR2_LEGACY_CHECK_2012_11
+
+#
+# Legacy options from 2012.08
+#
+
+endif

Makefile

@@ -290,6 +290,12 @@ endif
 
 all: world
 
+# Include legacy before the other things, because package .mk files
+# may rely on it.
+ifneq ($(BR2_DEPRECATED),y)
+include Makefile.legacy
+endif
+
 include package/Makefile.in
 include support/dependencies/dependencies.mk
 
@@ -762,4 +768,3 @@ print-version:
 include docs/manual/manual.mk
 
 .PHONY: $(noconfig_targets)
-

Makefile.legacy

@@ -0,0 +1,13 @@
+#
+# Makefile.legacy - support for backward compatibility
+#
+# This file contains placeholders to detect backward-compatibility problems.
+# When a buildroot "API" feature is being deprecated, a rule should be added
+# here that issues an error when the old feature is used.
+#
+# This file is not included if BR2_DEPRECATED is selected, so it is possible
+# to bypass the errors (although that's usually a bad idea).
+
+ifeq ($(BR2_LEGACY),y)
+$(error "You have legacy configuration in your .config! Please check your configuration.")
+endif