patches/openwrt: modify generic config to compile in BTRFS

BTRFS in default is not compiled in and we use it as root-FS so it
should be.

patches/openwrt/branding/0012-target-linux-generic-enable-BTRFS.patch

+From 50c98190c3e82b93d79c81e29921b19f34f9ef24 Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Karel=20Ko=C4=8D=C3=AD?= <karel.koci@nic.cz>
+Date: Thu, 2 Jan 2020 14:38:01 +0100
+Subject: [PATCH] target/linux/generic: enable BTRFS
+
+Turris routers use BTRFS as root filesystem. This enables is globally
+for all Turris OS builds on Linux 4.14.
+---
+ target/linux/generic/config-4.14 | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/target/linux/generic/config-4.14 b/target/linux/generic/config-4.14
+index b0805b1..2961b23 100644
+--- a/target/linux/generic/config-4.14
++++ b/target/linux/generic/config-4.14
+@@ -639,7 +639,7 @@ CONFIG_BROKEN_ON_SMP=y
+ # CONFIG_BT is not set
+ # CONFIG_BTRFS_ASSERT is not set
+ # CONFIG_BTRFS_DEBUG is not set
+-# CONFIG_BTRFS_FS is not set
++CONFIG_BTRFS_FS=y
+ # CONFIG_BTRFS_FS_POSIX_ACL is not set
+ # CONFIG_BTRFS_FS_RUN_SANITY_TESTS is not set
+ # CONFIG_BT_ATH3K is not set
+-- 
+2.24.1
+