v01d opened a new pull request #1786:
URL: https://github.com/apache/incubator-nuttx/pull/1786


   ## Summary
   
   This works by having the build system look for nuttx/external/Kconfig
   to determine whether this directory is present or not. nuttx/external
   is gitignored in order to be added by the final user but not to be
   commited into the repo. Tipically this will by a symbolic link, just like
   apps/external.
   
   Inside external/ a Makefile should be placed with the same structure
   than any nuttx/ subdirectory (eg: nuttx/drivers/). The
   nuttx/external/Kconfig will be sourced and any options defined there will
   appear at the bottom of menuconfig (unless options are conditioned on
   menus, in which case they will appear accordingly).
   
   The purpose is to allow arch/board independent code, which for any
   reason is not to be upstreamed (propietary, not relevant for mainline,
   testing, etc), to be built into the OS during OS building stage. This
   way the user does not need to fork the NuttX repo to do so. This feature
   complements well with external apps and custom board support.
   
   ## Impact
   
   This is implemented by extending the build to consider one extra subdirectory
   `external` depending on if `external/Kconfig` is present. The build process 
for
   this directory will be the same as for any of the nuttx top-level 
subdirectories.
   
   There's one tricky part which is that as Kconfig cannot conditionally 
include a
   file, in order to include include external/Kconfig when it exists, a similar 
strategy
   that is used for including ../apps/Kconfig is used. This works by defining 
an environment
   variable `EXTERNALDIR` which is set to `external` (when external/Kconfig 
exists)
   or to `.external-dummy` (if not). Inside this directory there's an empty 
Kconfig which
   will then be included when `external` is not present.
   This strategy avoids the need for autogenerated Kconfig file, as it is done 
for `apps/Kconfig`
   and thus I think it is a bit cleaner.
   
   ## Testing
   
   I tested building either with external and without it and it works 
correctly. Also,
   Kconfig options are exposed as expected.
   
   


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