Enable DIRECT_IO kernel config

Fixes a kernel incompatiblity with the upstream OpenWrt modules regarding
filesystem drivers (and probably other modules related to block devices).

include/config

@@ -15,6 +15,8 @@ CONFIG_BUSYBOX_CONFIG_FEATURE_IP_RULE=y
 CONFIG_BUSYBOX_CONFIG_FEATURE_IP_SHORT_FORMS=y
 CONFIG_BUSYBOX_CONFIG_FEATURE_WGET_TIMEOUT=y
 
+CONFIG_KERNEL_DIRECT_IO=y
+
 CONFIG_ATH_USER_REGD=y
 CONFIG_PACKAGE_ATH_DEBUG=y
 CONFIG_ATH10K_CT_COMMUNITY_FW=y