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/button_upper.c

@@ -56,6 +56,7 @@
 #include <nuttx/kmalloc.h>
 #include <nuttx/fs/fs.h>
 #include <nuttx/input/buttons.h>
+#include <nuttx/random.h>
 
 #include <nuttx/irq.h>
 
@@ -317,6 +318,8 @@ static void btn_sample(FAR struct btn_upperhalf_s *priv)
   DEBUGASSERT(lower->bl_buttons);
   sample = lower->bl_buttons(lower);
 
+  add_ui_randomness(sample);
+
 #if !defined(CONFIG_DISABLE_POLL) || !defined(CONFIG_DISABLE_SIGNALS)
   /* Determine which buttons have been newly pressed and which have been
    * newly released.