net: Enable context buffer pool in case TCP and 6LO are enabled

The context buffer pool was introduced to deal with TCP holding buffers
when 6LO may modify them, therefore it makes sense to have it enabled
by default when TCP and 6LO are enabled given that the code can deal
with NULL pool in case the application don't implement one.

Change-Id: I600ca31ab40c96ee27937c2e885e332b0cee4995
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>