support: move package/gnuconfig to support/gnuconfig

The CONFIG_UPDATE macro is no longer defined in
package/gnuconfig/gnuconfig.mk, but instead in
package/Makefile.autotools.in. It it also changed a little bit to take
the directory of the package sources as argument, and the AUTOTARGETS
infrastructure is updated to use this macro.

[Peter: drop echo in CONFIG_UPDATE]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

support/gnuconfig/README.buildroot

@@ -0,0 +1,25 @@
+--- HOWTO ---
+If your package uses config.guess and/or config.sub, then it probably
+relies on the autotools as its build system. In this case, you should
+use the AUTOTARGETS infrastructure, which will take care of updating
+the config.guess and/or config.sub files appropriately. See the
+Buildroot documentation for details about the AUTOTARGETS
+infrastructure.
+
+If for some reason your package does not use the AUTOTARGETS
+infrastructure, you can request the config.guess and/or config.sub
+files of your package to be updated by using:
+
+      $(call CONFIG_UPDATE,directory-of-your-package-sources)
+
+--- UPDATE ---
+GNU config is now managed in git, so to update:
+
+# git clone git://git.savannah.gnu.org/config.git
+# cp config/config.* .
+# for p in $(ls patches/*.patch); do patch -p0 < $p; done
+# rm -rf config
+
+A few local customizations are used to support uClibc so you may
+have to make sure they're still needed.  The patches are broken
+out in the patches/ dir to keep things simple.