UefiPayloadPkg: Add library for logging to EC

Make use of the SMFI command interface to forward logs from edk2 to
System76 EC.

UefiPayloadPkg/Include/Library/System76EcLib.h

@@ -0,0 +1,26 @@
+/** @file
+  System76 EC logging
+
+  Copyright (c) 2020 System76, Inc.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#ifndef __SYSTEM76_EC_LIB__
+#define __SYSTEM76_EC_LIB__
+
+/**
+  Write data to the embedded controller using SMFI command interface.
+
+  @param  Buffer            Pointer to the data buffer to be written.
+  @param  NumberOfBytes     Number of bytes to write.
+
+  @return -1 if the command failed, else the number of data bytes written.
+**/
+INTN
+System76EcWrite (
+  IN UINT8    *Buffer,
+  IN UINTN    NumberOfBytes
+);
+
+#endif /* __SYSTEM76_EC_LIB__ */