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>