net: buf: Add complete net_buf_simple implementation

This just renames the existing net_buf helper APIs to corresponding
net_buf_simple APIs and then defines the old net_buf API names as
macros mapping to the new net_buf_simple helpers.

Change-Id: I3c2b35683c0fde66f5f697be2c34c01172af8e31
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>