net: buf: Introduce variable data length allocators

Introduce two new "standard" data allocators to net_buf. There are now
three in total:

NET_BUF_POOL_FIXED_DEFINE: This is the closes to the old
implementation, i.e. fixed size chunks.  It's also what the old
NET_BUF_POOL_DEFINE macro maps to.

NET_BUF_POOL_HEAP_DEFINE: uses the OS heap

NET_BUF_POOL_VAR_DEFINE: defines a variable sized allocator using
k_mem_pool (this is all that there was in my first draft of this
feature)

Currently the variable length allocators (HEAP & VAR) support
reference counted data payloads, i.e. cheap cloning. The FIXED
allocator does not currentlty support this to allow for the simplest
possible implementation, but the support can be added later if
desired.

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