net: buf: Introduce net_buf_simple object

The object has the same layout as the last fields of the existing
net_buf. To maintain backwards compatibility net_buf receives a union
so that the old data, len and size members can be accessed as before,
however they are located in the same memory as the actual
net_buf_simple members.

Change-Id: I22d208faba30fe41c5ada17e6d13e641f528729a
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>