sha256: add optional support for kernel crypto API

[WallaceIT]

Add the possibility to enable support for the Linux kernel crypto API, which allows to perform the SHA256 operations exploiting the facilities exposed by the kernel, including hardware accelerators.

[WallaceIT]

On a Beagleplay (based on AM62 ARMv8 SoC), this lead to the following results:

Image size (bytes)	Compression algorithm	Write time with SW SHA256 (s)	Write time with kcapi SHA256 (s)	Write time without verification (s)
806473728	Zstandard	29.18s	21.15s	18.97s

[embetrix]

@WallaceIT : this is a great enhancement, however I have some suggestions:

can we add a runtime check if the kernel support AF_ALG ?

https://stackoverflow.com/questions/36074001/detect-availability-of-linux-kernels-af-alg-sockets-for-userland-crypto

If not fallback to normal mode ( I think we can even enable it by default)

[WallaceIT]

Hi, the check is already taken care by the libkcapi library through the kcapi_md_init function, which will fail the initialization if the AF_ALG interface or the hasing algorithm is not supported.

The automatic fallback is already there too: if the initialization fails, the init function will nevertheless return 0 and subsequent functions will use the SW implmentation:

int sha256Init(SHA256Ctx& context) {
    if (!context.initialized) {
#ifdef USE_KERNEL_CRYPTO_API
        const char *hashname = "sha256";
        static bool warned = false;
        int ret;

        ret = kcapi_md_init(&context.handle, hashname, 0);
        if ((ret != 0) && !warned) {
            std::cerr << "Failed to init kernel crypto API: " << ret << std::endl;
            std::cerr << "Falling back to software hashing" << std::endl;
            warned = true;
        }
#endif
        context.initialized = true; // <- Here the context gets marked as initialized even in case of failure
    }

    return 0;
}