lantiq: kernel 5.4: enable JFFS2

We do not build images with a jffs2 rootfs, but jffs2 is still used as
filesystem for the rootfs_data.

Fixes: e3eaf57808 ("lantiq: kernel 5.4: debloat kernel config")
Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/lantiq/config-5.4

@@ -129,7 +129,6 @@ CONFIG_IRQ_DOMAIN_HIERARCHY=y
 CONFIG_IRQ_FORCED_THREADING=y
 CONFIG_IRQ_MIPS_CPU=y
 CONFIG_IRQ_WORK=y
-# CONFIG_JFFS2_FS is not set
 CONFIG_LANTIQ=y
 CONFIG_LANTIQ_DT_NONE=y
 # CONFIG_LANTIQ_ETOP is not set