core/net/uip.h: Add missing #include <string.h>

Fixes implicit definition of memcmp() warnings/errors.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
Merge pull request #711 from gebart/upstream-fix-implicit-definitions

uip.h missing #include <string.h>
Merges Z1SP into Z1 platform
Only wait for a keypress if the user is told to press a key.
Merge pull request #747 from oliverschmidt/master

Only wait for a keypress if the user is told to press a key.
Merge pull request #739 from alignan/merge_z1sp_with_z1

Merges Z1SP into Z1 platform
Merge commit '784b20c04d299ccb7a7fb9d3c90aa31dff3bf520' into HEAD