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

pio/ir_nec/nec_receive_library/nec_receive.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_rx_init (PIO pio, uint pin);
-bool nec_decode_frame (uint32_t sm, uint8_t *p_address, uint8_t *p_data);
+
+int nec_rx_init(PIO pio, uint pin);
+bool nec_decode_frame(uint32_t sm, uint8_t *p_address, uint8_t *p_data);