Add support for ECDSA P-256 with SHA256

This adds a small EC library that is capable of verifying a signature of
SHA256 with ECDSA on the NIST P-256 curve.

Change-Id: I2a16639c92a77e8e4783c47ffbc56676de56eb59
diff --git a/include/mincrypt/rsa.h b/include/mincrypt/rsa.h
index cc0e800..3d0556b 100644
--- a/include/mincrypt/rsa.h
+++ b/include/mincrypt/rsa.h
@@ -25,8 +25,8 @@
 ** ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
-#ifndef _EMBEDDED_RSA_H_
-#define _EMBEDDED_RSA_H_
+#ifndef SYSTEM_CORE_INCLUDE_MINCRYPT_RSA_H_
+#define SYSTEM_CORE_INCLUDE_MINCRYPT_RSA_H_
 
 #include <inttypes.h>
 
@@ -55,4 +55,4 @@
 }
 #endif
 
-#endif
+#endif // SYSTEM_CORE_INCLUDE_MINCRYPT_RSA_H_