luci-mod-network: Can't configure tunnel GRE interface attributes

[tiagogaspar8]

Steps to reproduce:

1. Create a GRE interface in Network → Interfaces
2. go to Network → Interfaces → Devices
3. No GRE interface is shown on the list
4. Select "Add device configuration"
5. Try to select the GRE tunnel in the "Existing Device" section
6. The GRE tunnel isn't listed

Actual behavior:

The GRE tunnel isn't seen as a configurable device

1. No GRE interface is shown on the list in Network → Interfaces → Devices
2. The GRE tunnel isn't listed in "Add device configuration"

Expected behavior:

The GRE tunnel is able to be configured in the mentioned sections.

Additional Information:

OpenWrt version information from system /etc/openwrt_release

DISTRIB_ID='OpenWrt'
DISTRIB_RELEASE='11' #my versioning based on master
DISTRIB_REVISION='r26386-1188cae092'
DISTRIB_TARGET='ath79/generic'
DISTRIB_ARCH='mips_24kc'
DISTRIB_DESCRIPTION='OpenWrt 11 r26386-1188cae092'
DISTRIB_TAINTS='no-all'

One example of a option that I'd like to configure is /proc/sys/net/ipv4/conf/gre4-gre/accept_local for the GRE tunnel, which is available in '/proc' but not in the LuCI interface.