sys/shell: fix error when not using sixlowpan

```
/sys/shell/commands/sc_net_if.c:631:27: error: ‘IPV6_MAX_ADDR_STR_LEN’ undeclared (first use in this function)
             char addr_str[IPV6_MAX_ADDR_STR_LEN];
                           ^
```
make: model net_if dependency on net_help
make: replace spaces with tabs in Makefile.dep
Merge pull request #1401 from LudwigOrtmann/scnetif_error

sys/net_if, make: fixup