Some public API functions do not modify the net_buf instances and can
declare these arguments as const.
Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>
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 <mariusz.skamra@codecoup.pl>