doc/doxygen: add build-system-basics.md

doc/doxygen/riot.doxyfile

@@ -766,6 +766,7 @@ INPUT                  = ../../doc.txt \
                          src/creating-an-application.md \
                          src/getting-started.md \
                          ../../tests/README.md \
+                         src/build-system-basics.md \
                          src/advanced-build-system-tricks.md \
                          src/changelog.md \
                          ../../LOSTANDFOUND.md

doc/doxygen/src/build-system-basics.md

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+# Build System Basics                                       {#build-system-basics}
+
+# Variables declaration guidelines              {#variable-declaration-guidelines}
+
+This page contains basic guidelines about `make` variable declaration, it
+summarizes some of the pros and cons as well as specifies good and bad patterns
+in our build system. You might want to refer to `gnu make` documentation
+regarding these subjects.
+
+## Avoid unnecessary export
+
+```
+export OUTPUT = $(shell some-command)
+```
+
+Exporting a variable means it will be evaluated on every `target` call, which
+slows down the build system. Always avoid exporting a variable if unneeded.
+
+If an export is actually needed by a `sub-make` then export the variable only for
+the needed targets using `target-export-variables` (more in
+`makefiles/utils/variables.mk`).
+
+Exported variables ("global variable") are hard to remove, specially when badly
+documented. If no one knows why it's there and no one knows where it can be used
+then no one knows if it's safe to remove since it's present for every target.
+This is why global variables need clear documentation.
+
+[gnumake doc](https://www.gnu.org/software/make/manual/html_node/Variables_002fRecursion.html)
+
+## Use memoized for variables referencing a function or command
+
+### recursively expanded variable:
+
+~~~~~~~~~~~~~~~~
+OUTPUT = $(shell some-command $(ANOTHER_VARIABLE))
+~~~~~~~~~~~~~~~~
+
+- When using `=` the value of the variable is only declared, but not set,
+  therefore the variable will only be evaluated when expanded (used) somewhere
+  in the makefile. If `$(OUTPUT)` is never expanded, `some-command`
+  is never executed and `ANOTHER_VARIABLE` not expanded.
+
+- All variables or functions referenced by the declared variable will will be
+  evaluated every time the variable is expanded.
+  In the example `some-command` is executed every time `OUTPUT` is expanded, same for
+  `ANOTHER_VARIABLE`. If `some-command` is slow this introduced unneeded overhead.
+
+- If the variable expansion doesn't involve evaluating a function the overhead
+  is none.
+
+### simply expanded variable:
+
+~~~~~~~~~~~~~~~~
+OUTPUT := $(shell some-command $(ANOTHER_VARIABLE))
+~~~~~~~~~~~~~~~~
+
+- When using `:=` the value is only expanded once, expanding any reference to
+  other variables or functions. If `OUTPUT` is always used at least once and
+  evaluates a costly function (`some command`) then use `:=`.
+
+- When using `:=` the variable will be evaluated even if not needed, which
+  introduces unnecessary delay, in particular `some command` or functions
+  evaluated by `ANOTHER_VARIABLE` are slow.
+  It can also cause a failure in a worst-case scenario (think what happens if a
+  tool is defined with `:=` but you don't have the tool and you don't need it either).
+
+- The values of variables declared with `:=` depend on the order of definition.
+
+### memoized:
+
+~~~~~~~~~~~~~~~~
+OUTPUT = $(call memoized,OUTPUT,$(shell some-command))
+~~~~~~~~~~~~~~~~
+
+- `memoized` is a RIOT defined function that combines characteristics from
+  both `=` and `:=`.
+  The variable expansion will be deferred until its first usage, but further
+  usage will consider it as a simply expanded variable, so it will use the already
+  evaluated value. In the example `some-command` would be executed once or not
+  at all (more in `makefiles/utils/variables.mk`).
+
+[gnumake doc](https://www.gnu.org/software/make/manual/html_node/Flavors.html)
+
+## Additional documentation
+
+- Deferred vs. simple expansion: http://make.mad-scientist.net/deferred-simple-variable-expansion/
+- Tracking issue: [#10850](https://github.com/RIOT-OS/RIOT/issues/10850)