package/docker-engine: fix btrfs handling

btrfs doesn't depend on btrfs-progs but on headers >= 4.12 since bump to
version 24.0.2 in commit 314f62eda347ba02b4688ac94c6bab55b4a4d6c9 and
https://github.com/moby/moby/commit/3208dcabdc8997340b255f5b880fef4e3f54580d
resulting in the following build failure:

daemon/graphdriver/btrfs/btrfs.go:13:6: error: #error "Headers from kernel >= 4.12 are required to build with Btrfs support."
     #error "Headers from kernel >= 4.12 are required to build with Btrfs support."
      ^~~~~
daemon/graphdriver/btrfs/btrfs.go:14:6: error: #error "HINT: Set 'DOCKER_BUILDTAGS=exclude_graphdriver_btrfs' to build without Btrfs."
     #error "HINT: Set 'DOCKER_BUILDTAGS=exclude_graphdriver_btrfs' to build without Btrfs."
      ^~~~~
daemon/graphdriver/btrfs/btrfs.go:18:10: fatal error: linux/btrfs_tree.h: No such file or directory
 #include <linux/btrfs_tree.h>
          ^~~~~~~~~~~~~~~~~~~~

Fixes: 314f62eda347ba02b4688ac94c6bab55b4a4d6c9
 - http://autobuild.buildroot.org/results/7d07eba37149d341dc86f9742bd166de874dcd5e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit e30b38f1c5bcc62d387f4b96d20a7a9f4715c7a5)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/docker-engine/Config.in

@@ -21,12 +21,13 @@ if BR2_PACKAGE_DOCKER_ENGINE
 
 config BR2_PACKAGE_DOCKER_ENGINE_DRIVER_BTRFS
 	bool "btrfs filesystem driver"
-	depends on BR2_USE_MMU # btrfs-progs
-	depends on BR2_TOOLCHAIN_HAS_THREADS # btrfs-progs
-	select BR2_PACKAGE_BTRFS_PROGS
+	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_12
 	help
 	  Build the btrfs filesystem driver for Docker.
 
+comment "brtfs filesystem driver needs headers >= 4.12"
+	depends on !BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_12
+
 config BR2_PACKAGE_DOCKER_ENGINE_DRIVER_DEVICEMAPPER
 	bool "devicemapper filesystem driver"
 	depends on BR2_TOOLCHAIN_HAS_THREADS # lvm2

package/docker-engine/docker-engine.mk

@@ -35,9 +35,7 @@ DOCKER_ENGINE_DEPENDENCIES += systemd
 DOCKER_ENGINE_TAGS += systemd journald
 endif
 
-ifeq ($(BR2_PACKAGE_DOCKER_ENGINE_DRIVER_BTRFS),y)
-DOCKER_ENGINE_DEPENDENCIES += btrfs-progs
-else
+ifneq ($(BR2_PACKAGE_DOCKER_ENGINE_DRIVER_BTRFS),y)
 DOCKER_ENGINE_TAGS += exclude_graphdriver_btrfs
 endif