fix rpl_find_preferred_parent
Merge pull request #361 from LudwigOrtmann/rpl_find_preferred_parent

fix rpl_find_preferred_parent
get rid of static buffer

this buffer was used for two types of outgoing packets:
  1. local msg
    - msg can be big, there is is no apriori boundary -> use dynamic
      memory instead
  2. transceiver msg
    - no need to copy msg in this buffer -> transceiver send is blocking
tune CCNL_RIOT_CHUNK_SIZE

this constant is used for two test cases:

1. populate + interest /riot/text
  -> static content has this=90 bytes size

2. appserver + interest /riot/appserver/test
  -> dynamic content is created with exact
     CCNL_RIOT_CHUNK_SIZE bytes
  -> CCNL_RIOT_CHUNK_SIZE can be any value the
     transceiver can handle
make max_cache_entries a parameter
fix: remove oldest dynamic entry in content store
enable timeout events