projects: drop CFLAGS explicit initialization

[amiclaus]

Most of the CFLAGS from no-OS projects are used within the code by checking if they are defined. Therefore explicit initialization is redundant. Drop it.

Pull Request Description

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[buha]

We really should work on something like EXAMPLE=basic or EXAMPLE=iio etc. in the future because examples are mutually exclusive and currently we have a lot of crazy stuff like this: https://github.com/analogdevicesinc/no-OS/blob/main/projects/eval-adxl355-pmdz/src/platform/aducm3029/main.c#L115-L120