build: add comments to makefiles

Add comments to build system makefile functions and variables to help
developers in understanding build system internals and ease the
development process.

This patch adds some documentation examples with proposed doxygen-like
syntax. Hopefully, this would start the discussion and result in
generation of the makefile documentation guidelines.

Signed-off-by: Sergey Matsievskiy <matsievskiysv@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16888


Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit a72c8c77)
Link: https://github.com/openwrt/openwrt/pull/17097


Signed-off-by: Petr Štetiar <ynezz@true.cz>