libpayload: Provide dummy getenv()

Change-Id: I419fcb16e0b10dee9195072e0e6befa6c9e61a69
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/105
Tested-by: build bot (Jenkins)
Reviewed-by: Frank Vibrans III <frank.vibrans@amd.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/libc/lib.c

@@ -120,3 +120,9 @@ void exit(int status)
 }
 
 int errno;
+
+char *getenv(const char *name)
+{
+	return NULL;
+}
+