Make configuration easier by including user_config.h

[thierer]

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By including user_config.h at the top of the file, it's possible to make configuration changes while using the unmodified code from the repository.

To make that also possible for both PWM_DEBUG and PWM_USE_NMI, include them in ifndef like already done for SDK_PWM_PERIOD_COMPAT_MODE and PWM_MAX_CHANNELS.