netifd: add devtype to ubus call

netifd: add devtype to ubus call

Every network device has a type but there is no standard interface here.
The type can be determined either from the file
'/sys/class/net/<device>/uevent' or, if no information is found
there, from the file '/sys/class/net/<device>/type'.

This new function first checks whether there is a DEVTYPE=<type> string
in the 'uevent' file and uses it. If it does not find this information,
the 'type' is used as a fallback and mapped the number to a character
sequence.

This new 'devtype' information can be found in the network.device ubus
call.

Command:
ubus call network.device status

Output:
{
    "eth0": {
        "devtype": "ethernet",

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com> [commit rewording]