tools: generic.mk: add NO_OS_INC_DIRS build var for pulling header files

[commodo]

Pull Request Description

This is inspired by the SRC_DIRS build variable, but only focuses on include files. Using the SRC_DIRS can be problematic, as it pulls every .c & .h file from a directory.

But in some cases, we just want to pull in the header files. These header files will only get compiled if a .c file pulls them in.

If this build variable is unspecified, it will be empty, so nothing will be done (to potentially affect current projects).

PR Type

• [ ] Bug fix (change that fixes an issue)
• [x] New feature (change that adds new functionality)
• [ ] Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• [x] I have followed the Coding style guidelines
• [x] I have performed a self-review of the changes
• [x] I have commented my code, at least hard-to-understand parts
• [x] I have build all projects affected by the changes in this PR
• [x] I have tested in hardware affected projects, at the relevant boards
• [x] I have signed off all commits from this PR
• [x] I have updated the documentation (wiki pages, ReadMe etc), if applies

[commodo]

Changelog v1 -> v2:

• renamed INC_DIRS -> NO_OS_INC_DIRS ; Mbed seems to have an INC_DIRS variable and this seems to collide with it in some builds