It's the recommended way to set those, and is more portable
(automatically prepends -D for GCC/Clang and /D for MSVC).
We still use CPPFLAGS for some pre-processor flags which are not
defines.
Godot core needs MD5/SHA256/AES/Base64 which used to be provided by
separate libraries.
Since we bundle mbedtls in most cases, and we can easily only include
the needed sources if we so desire, let's use it.
To simplify library changes in the future, and better isolate header
dependencies all functions have been wrapped around inside a class in
`core/math/crypto_base.h`.
If the mbedtls module is disabled, we only bundle the needed source
files independently of the `builtin_mbedtls` option.
If the module is enabled, the `builtin_mbedtls` option works as usual.
Also remove some unused headers from StreamPeerMbedTLS which were
causing build issues.
