Revert "package/flannel: update project URL"

This reverts commit 9d46c05d1d5a3b5bf14ae5e3c80cba565b935496.

This breaks the build on the 2021.02.x branch:
http://autobuild.buildroot.net/results/8c2/8c271c09f3eb4711752c83446dc780bb4349d21e/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/flannel/Config.in

@@ -12,4 +12,4 @@ config BR2_PACKAGE_FLANNEL
 	  Flannel is a virtual network that gives a subnet to each
 	  host for use with container runtimes.
 
-	  https://github.com/flannel-io/flannel
+	  https://github.com/coreos/flannel

package/flannel/flannel.mk

@@ -5,13 +5,13 @@
 ################################################################################
 
 FLANNEL_VERSION = 0.12.0
-FLANNEL_SITE = https://github.com/flannel-io/flannel/archive
+FLANNEL_SITE = https://github.com/coreos/flannel/archive
 FLANNEL_SOURCE = v$(FLANNEL_VERSION).tar.gz
 
 FLANNEL_LICENSE = Apache-2.0
 FLANNEL_LICENSE_FILES = LICENSE
 
-FLANNEL_LDFLAGS = -X github.com/flannel-io/flannel/version.Version=$(FLANNEL_VERSION)
+FLANNEL_LDFLAGS = -X github.com/coreos/flannel/version.Version=$(FLANNEL_VERSION)
 
 # Install flannel to its well known location.
 define FLANNEL_INSTALL_TARGET_CMDS