net: Append device name to linker generated symbols for struct net_if

Before this change all struct net_if objects accessed by for example
net_if_get_by_index() are placed in one linker area (i.e. net_if_area)
with the same "name" - '_net_if.static.net_if'. This may cause problems
when the order of struct net_if elements is important.

With the same names for all elements there is no guarantee of placement
order. After this change the unique device name is appended, so
SORT_BY_NAME() linker command places objects in reproductible manner.

Signed-off-by: Lukasz Majewski <lukma@denx.de>