Cleanup 3rd party samples; update README.md; add some missing copyright, fix builds for boards without certain pin definitions

pio/ir_nec/nec_transmit_library/nec_transmit.h

@@ -1,7 +1,13 @@
+/**
+ * Copyright (c) 2021 mjcross
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
 #include "pico/stdlib.h"
 #include "hardware/pio.h"
 
 // public API
-//
+
 int nec_tx_init(PIO pio, uint pin);
-uint32_t nec_encode_frame (uint8_t address, uint8_t data);
+uint32_t nec_encode_frame(uint8_t address, uint8_t data);