native: remove stale TODOs and blank line

The values are quite OK.
native: enable reboot with tap

partially fixes #787
Merge pull request #1408 from LudwigOrtmann/native-cleaning

native: remove stale TODOs and blank line
Merge pull request #1410 from LudwigOrtmann/issue-787

native: enable reboot with tap
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