separate unittest folder

unittests/testcfg/tmtc/apid.h

@@ -0,0 +1,17 @@
+#ifndef CONFIG_TMTC_APID_H_
+#define CONFIG_TMTC_APID_H_
+
+#include <cstdint>
+
+/**
+ * Application Process Definition: entity, uniquely identified by an
+ * application process ID (APID), capable of generating telemetry source
+ * packets and receiving telecommand packets.
+ *
+ * Chose APID(s) for mission and define it here.
+ */
+namespace apid {
+static const uint16_t DEFAULT_APID = 0x00;
+}
+
+#endif /* CONFIG_TMTC_APID_H_ */