tools: fix the path generated to the sct file

The sct file path generated in the online compiler
is incorrect. Fix that by changing the correct_scatter_shebang
API to accept a FileRef object instead and use the path.

This change should go with change in online compiler that removes
the override for correct_scatter_shebang.

The Makefile is run from the build directory. The source files were
properly prefixed with "../", however the paths provided to the
assmebler were not. This ensures the assembler include paths are
prefixed properly.

CMake and makefile exporters share a common logic for hex file
selection. Factor it as a common property in the base class
to avoid code duplication.

Rename the existing PSoC-specific m0_core_img key in targets.json
as a more generic hex_filename key. Update makefile exporter to select
the subset of resources.hex_files matching the hex_filename value.
Without this fix, multiple prebuilt CM0+ hex files are found in the
target resources and erroneously passed to the srec_cat tool.

The fix is generic so other targets that need post-build hex merging
can use this key to pass the correct image to srecord tool.

The fix also removes sub_target key: instead, rely hex_filename json
key to detect if the hex image merging needs to be done.
The sub_target is not used in mbed-os codebase for anything else.

It is possible to override the hex file name in mbed_app.json:
{
  "target_overrides": {
    "*": {
      "target.hex_filename": "my_custom_m0_image.hex"
    }
}

In Python 3, the map() function returns a map object, not a list object as in
Python 2. Ensure a list object is returned from format_flags() by wrapping
map() in list(). This is compatible with both Python 2 and 3.

OSs don't agree on what the quote chars mean

This reverts commit a242799.

Tools: Include configuration in ASM

Support Nuvoton's NUMAKER_PFM_M2351 target

No longer supported in Py3.

Add include search path when preprocessing assembly files for uvision and make_armc5 exports

When exporting a mbed project to make_armc5, the include options (-I)
of the ASM flags are not pointing to the good folder. It should be
pointing to the root mbed-os folder and not the one in BUILD.
This issue was found in the pull request ARMmbed/mbed-os#6168.

Signed-off-by: Hugues de Valon <hugues.devalon@arm.com>

Python2+3: mbed compile, mbed test --compile,  python unit tests