uclibc: re-add uclibc-update-config target

Commit a5e92d92 converted uClibc to the
package infrastructure, but while doing so removed support for the
documented uclibc-update-config make target. This make target copies the
uclibc .config file back to the original (typically version-controlled)
configuration file.

As this was an unintentional removal, this patch re-adds the
uclibc-update-config target.

Reported-by: Dan Moulding <dan.moulding@rackwareinc.com>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>