Memory optimizations in signing (#874)

* Sha256 driver cosmetics

cosmetic cleanups:
- use actual define that was used to allocate the array
- remove unused file globals

* Save memory by eliminating sha256_hash array

eliminated sha256_hash array to save memory. Instead pass the
Sha256Class result pointer.

There seems to be no use for the extra sha256_hash array as it was
zeroed exactly at the same time, the Sha256Class result was invalidated
(by calling signerSha256Init)

If the result needs to stay around while another sha is calculated,
it's better to have the user taking care of this.

* Moved hmac methods and buffers to separate class

This allows saving the buffer memory if no hmac is needed ... eg. with
the public Signing API.

core/MySigningAtsha204Soft.cpp

@@ -63,7 +63,7 @@ static void buf2str(const uint8_t* buf, size_t sz)
 #define SIGN_DEBUG(x,...)
 #endif
 
-Sha256Class _signing_sha256;
+HmacClass _signing_sha256;
 static unsigned long _signing_timestamp;
 static bool _signing_verification_ongoing = false;
 static uint8_t _signing_verifying_nonce[32+9+1];