Build error when parameter is overridden in mbed_app.json (for mbed libraries into project/lib)

Configuration

Operating system: Ubuntu 16.04.6 LTS

PlatformIO Version (platformio --version): version 4.1.1b9

Description of problem

Currently it is possible to build mbed libraries in project/lib directory as PlatformIO can parse mbed_lib.json for each library and add macroses into mbed_config.h. The issue appears when you try to override parameters of those libraries through mbed_app.json. Build fails with error "mbed build API internal error. Attempt to override undefined parameter". It looks like currently it is only possible to override parameters of libraries which come with mbed by default.

Steps to Reproduce

1. clone https://github.com/edgrin/pio-mbed-lib-target-overrides/tree/a8b580c5deb6e36e74de38f5755cdb611166445f
2. try to build
3. see error Attempt to override undefined parameter 'newlib.param' in 'application[*]'
4. delete line 4 with '"newlib.param": 2' in mbed_app.json
5. try to build
6. see successful build with newlib in Dependency Graph

Actual Results

mbed build API internal error

Expected Results

Successful build