examples/i2schar: Implement loopback mode check

This commit implements the loopback mode for the i2schar example
application. This mode - available only when both RX and TX are
enabled - allows the user to test the I2S buses when the TX pin is
connected to the RX pin. This is done by pre-filling a buffer with
known data (additionally, it checks the peripheral's data width to
format the data in the buffer accordingly) and passing it for both
the transmitter and the receiver threads. This buffer is written to
the TX and the received buffer is compared to the known buffer.

Signed-off-by: Tiago Medicci <tiago.medicci@espressif.com>

Author: tmedicci

examples/i2schar/i2schar.h

@@ -73,7 +73,7 @@
 #endif
 
 #ifndef CONFIG_EXAMPLES_I2SCHAR_BUFSIZE
-#   define CONFIG_EXAMPLES_I2SCHAR_BUFSIZE 256
+#  define CONFIG_EXAMPLES_I2SCHAR_BUFSIZE 256
 #endif
 
 /****************************************************************************
@@ -83,6 +83,7 @@
 struct i2schar_state_s
 {
   bool      initialized;
+  bool      loopback;
   FAR char *devpath;
 #ifdef CONFIG_EXAMPLES_I2SCHAR_TX
   int       txcount;