Remove nfc-defines.h public header.

Use NULL instead of INVALID_DEVICE_INFO to know if device is valid.
Make distcheck OK

src/examples/nfcip-target.c

@@ -24,6 +24,8 @@
 #include <stdio.h>
 #include <nfc.h>
 
+#define MAX_FRAME_LEN 264
+
 int main(int argc, const char *argv[])
 {
   byte_t abtRecv[MAX_FRAME_LEN];