net: buf: Factor out net_buf_simple to separate source file

This moves net_buf_simple related code to separate source file.
Having those in separate file makes unit testing easier as simple
network buffers do not use kernel objects, thus can be used
in unit tests without a need for adding any mocks.

Signed-off-by: Mariusz Skamra <[email protected]>

Author: MariuszSkamra

subsys/net/CMakeLists.txt

@@ -1,7 +1,11 @@
 # SPDX-License-Identifier: Apache-2.0
 
 zephyr_library()
-zephyr_library_sources_ifdef(CONFIG_NET_BUF             buf.c)
+zephyr_library_sources_ifdef(
+  CONFIG_NET_BUF
+  buf.c
+  buf_simple.c
+  )
 zephyr_library_sources_ifdef(CONFIG_NET_HOSTNAME_ENABLE hostname.c)
 
 if(CONFIG_NETWORKING)