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/stmpe811_tsc.c

@@ -62,6 +62,7 @@
 #include <nuttx/fs/fs.h>
 #include <nuttx/i2c/i2c_master.h>
 #include <nuttx/wqueue.h>
+#include <nuttx/random.h>
 
 #include <nuttx/arch.h>
 #include <nuttx/input/touchscreen.h>
@@ -534,6 +535,8 @@ static ssize_t stmpe811_read(FAR struct file *filep, FAR char *buffer, size_t le
   report->point[0].y         = sample.y;
   report->point[0].pressure  = sample.z;
 
+  add_ui_randomness((sample.x << 16) ^ (sample.y << 8) ^ sample.z);
+
   /* Report the appropriate flags */
 
   if (sample.contact == CONTACT_UP)