Move macro definitions out of debug.h

[gilles-peskine-arm]

The public header file mbedtls/debug.h contains the declaration of mbedtls_debug_set_threshold, and many macro definitions. mbedtls_debug_set_threshold belongs in a public header file, but not the macro definitions. All the macro definitions are intended when calling a function to emit a debug message, and those functions aren't even declared in a public header.

The goal of this task is to move the macro definitions from include/mbedtls/debug.h to library/debug_internal.h.

[mpg]

See also #6859 with a slightly bigger scope.

Perhaps the cost/benefit analysis for renaming is not favourable (I like things to be tidy, but is it worth bothering existing users?) - but we should probably consider if we really still need a public debug.h or can just move mbedtls_debug_set_threshold() to ssl.h.

[gilles-peskine-arm]

Yes, I figure it's a lot easier to move a bunch of definitions than to rename a header file and update all the code and add the requisite ifdefs in mbedtls-framework and whatever else we'd need to do if we remove mbedtls/debug.h and rename mbedtls_debug_set_threshold to mbedtls_ssl_debug_set_threshold.

We could move the definition of mbedtls_debug_set_threshold to ssl.h and keep a debug.h that just includes ssl.h. But that doesn't seem particularly useful to me if we don't go bigger and also at least rename the function.