net: pkt: Use standard macros to define system pkt slabs/pools

Previously net_pkt.h, defined macros NET_PKT_TX_SLAB_DEFINE,
NET_PKT_DATA_POOL_DEFINE, but advertised them as intended for
"user specified data". However, net_pkt.c effectively used the
same parameters for slabs/pools, but this wasn't obvious due
to extra config param redirection. So, make following changes:

1. Rename NET_PKT_TX_SLAB_DEFINE() to NET_PKT_SLAB_DEFINE()
as nothing in its definition is TX-specific.
2. Remove extra indirection for config params, and use
NET_PKT_SLAB_DEFINE and NET_PKT_DATA_POOL_DEFINE to define
system pools.
3. Update docstrings for NET_PKT_SLAB_DEFINE and
NET_PKT_DATA_POOL_DEFINE.

Overall, this change removes vail of magic in the definition of
system pkt slabs/pools, making obvious the fact that any packet
slabs/pools - whether default system or additional, custom - are
defined in exactly the same manner (and thus work in the same manner
too).

Fixes #4327

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
2 files changed