Defining CMake variables from the command-line or from another CMake project,
such as `-DCMAKE_C_FLAGS= -DCMAKE_CXX_FLAGS=`, caused a link failure as ESP
CMake was unable to set its proper compilation flags.
Additional CMake compiler flags can now be provided by another project.
* Closes https://github.com/espressif/esp-idf/issues/7507
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.
Refactor IDF "project" functionality under a wrapping of the default
"project" command, so we can tweak it a bit...
Will need more testing in other environments.
morris