Add iface_max_route_count-0 feature flag

In certain configurations interface can work without route table (for example,
interface with L3 IP medium). In this configuration it's desirable to reduce space
reserved by route table to 0 entries to reclaim memory. Currently it's impossible -
minimum route table size is 1 entry (feature `iface_max_route_count-1`).

I'm adding a feature `iface_max_route_count-0` which reduces size of route table to 0 entries.

Cargo.toml

@@ -172,6 +172,7 @@ iface-neighbor-cache-count-256 = []
 iface-neighbor-cache-count-512 = []
 iface-neighbor-cache-count-1024 = []
 
+iface-max-route-count-0 = []
 iface-max-route-count-1 = []
 iface-max-route-count-2 = [] # Default
 iface-max-route-count-3 = []

gen_config.py

@@ -31,7 +31,7 @@ feature("iface_max_addr_count", default=2, min=1, max=8)
 feature("iface_max_multicast_group_count", default=4, min=1, max=1024, pow2=8)
 feature("iface_max_sixlowpan_address_context_count", default=4, min=1, max=1024, pow2=8)
 feature("iface_neighbor_cache_count", default=8, min=1, max=1024, pow2=8)
-feature("iface_max_route_count", default=2, min=1, max=1024, pow2=8)
+feature("iface_max_route_count", default=2, min=0, max=1024, pow2=8)
 feature("fragmentation_buffer_size", default=1500, min=256, max=65536, pow2=True)
 feature("assembler_max_segment_count", default=4, min=1, max=32, pow2=4)
 feature("reassembly_buffer_size", default=1500, min=256, max=65536, pow2=True)