Merge branch 'feature/build_component_project_vars' into 'master'

build system: General refactoring

The original goal here was to support setting different COMPONENT_INCLUDEDIRS/COMPONENT_LDFLAGS based on sdkconfig. It's turned into a general refactor.

* The `get_variable` target & associated macros have been replaced with a target that generates a makefile snippet for each component (inside the build directory), and includes these in the project make pass. This improves build speed ( "no-op" `make all` on my system is down from 1.43s  (wall) / 0.55s (CPU) to 0.49s (wall) / 0.23s (CPU) due to not forking as many processes) and also makes the builds more debuggable, as you can go and look at the component_project_vars.mk files in the build directory to see what paths are being added where.
* Including component_common.mk from component.mk is no longer necessary (and prints a deprecation warning). Instead, component.mk is included from inside a wrapper makefile. This allows default variables and project configuration to be loaded before component.mk begins, and removes the need to split makefile statements arbitrarily between "before including component_common" and "after including component_common", with which goes where depending on internals of the build system. Part-fix for TW#8017.
* Rewrote build system docs to reflect these details and also clarify some other points, add more ReST structure. Fixes other issues raised in TW#8017.
* Should be backwards compatible with existing components. The only difference I know is the deprecation warning when including component_common.mk.

Adds one new failure case: because some paths are encoded in the generated component_project_vars.mk files then reorganising the component directory structure (but not modifying any other files or menuconfig) may cause a partial build to fail and require a `make clean`. Moving the whole project directory, whole build directory, or whole ESP-IDF directory will not trigger this. This failure case should only ever cause a build error, not incorrect build output.

See merge request !188