make: allow tagged RIOT_VERSION
make: `term` depends on `flash`

If you provide both `flash` and `term` as Make goals, then `term` needs
to wait for flash.
ip: check for null before ip_get_next_hop is used
Merge pull request #2124 from Kijewski/all-flash-term

make: term depends on flash
Merge pull request #1913 from Kijewski/issue-1910

make: allow tagged RIOT_VERSION
Merge pull request #2178 from cgundogan/ip_fix_null_check

ip: check for null before ip_get_next_hop is used