Add entropy pool and strong random number generator

Entropy pool gathers environmental noise from device drivers, user-space, etc., and returns good random numbers, suitable for cryptographic use. Based on entropy pool design from *BSDs and uses BLAKE2Xs algorithm for CSPRNG output.

Patch also adds /dev/urandom support for using entropy pool RNG and new 'getrandom' system call for getting randomness without file-descriptor usage (thus avoiding file-descriptor exhaustion attacks). The 'getrandom' interface is similar as 'getentropy' and 'getrandom' available on OpenBSD and Linux respectively.

drivers/input/mxt.c

@@ -64,6 +64,7 @@
 #include <nuttx/fs/fs.h>
 #include <nuttx/i2c/i2c_master.h>
 #include <nuttx/wqueue.h>
+#include <nuttx/random.h>
 
 #include <nuttx/semaphore.h>
 #include <nuttx/input/touchscreen.h>
@@ -889,6 +890,8 @@ static void mxt_touch_event(FAR struct mxt_dev_s *priv,
       sample->pressure = pressure;
       sample->valid    = true;
 
+      add_ui_randomness((x << 16) ^ y ^ (area << 9) ^ (pressure << 1));
+
       /* If this is not the first touch report, then report it as a move:
        * Same contact, same ID, but with a new, updated position.
        * The CONTACT_REPORT state means that a contacted has been detected,