How idf_build_component and the COMPONENTS argument to idf_build_process
interact is not clear/misleading. Clarify their interaction in the docs.
Closes: https://github.com/espressif/esp-idf/issues/3630
!4452 used setting LINK_LIBRARIES and INTERFACE_LINK_LIBRARIES to link
components built under ESP-IDF build system. However, LINK_LIBRARIES does
not produce behavior same as linking PRIVATE. This MR uses the new
signature for target_link_libraries directly instead. This also moves
setting dependencies during component registration rather than after all
components have been processed.
The consequence is that internally, components have to use the new
signature form as well. This does not affect linking the components to
external targets, such as with idf_as_lib example. This only affects
linking additional libraries to ESP-IDF libraries outside component processing (after
idf_build_process), which is not even possible for CMake<v3.13 as
target_link_libraries is not valid for targets not created in current
directory. See https://cmake.org/cmake/help/v3.13/policy/CMP0079.html#policy:CMP0079
!4452 simplified early expansion by using an early expansion script that
only does one thing: get the public and private requirements for each
component, albeit one by one. This was also dependent on parsing
the command output of the expansion script. This commit makes it so that a list of all
components to be processed to passed to the expansion script, generating a cmake
file that sets each component requirements in one go.
This also makes sure that only components that registered themselves get
included in the final build list.
!4452 had config generation first before building the component list
to be used in the build. This proved to be detrimental when a new target
is added as config generation would consider configs from both targets.
Removes the need to know/guess the paths to these libraries. Once we are gcc 8 only, we
can remove -nostdlib and no additional arguments are needed for system libraries.
The catch is: any time IDF overrides a symbol in the toolchain sysroot, we need
an undefined linker marker to make sure this symbol is seen by linker.
