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/ads7843e.c

@@ -70,6 +70,7 @@
 #include <nuttx/fs/fs.h>
 #include <nuttx/spi/spi.h>
 #include <nuttx/wqueue.h>
+#include <nuttx/random.h>
 
 #include <nuttx/semaphore.h>
 #include <nuttx/input/touchscreen.h>
@@ -624,6 +625,8 @@ static void ads7843e_worker(FAR void *arg)
       y = ads7843e_sendcmd(priv, ADS7843_CMD_YPOSITION);
 #endif
 
+      add_ui_randomness((x << 16) | y);
+
       /* Perform a thresholding operation so that the results will be more stable.
        * If the difference from the last sample is small, then ignore the event.
        * REVISIT:  Should a large change in pressure also generate a event?