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.

crypto/Makefile

@@ -56,6 +56,18 @@ ifeq ($(CONFIG_CRYPTO_SW_AES),y)
   CRYPTO_CSRCS += aes.c
 endif
 
+# BLAKE2s hash algorithm
+
+ifeq ($(CONFIG_CRYPTO_BLAKE2S),y)
+  CRYPTO_CSRCS += blake2s.c
+endif
+
+# Entropy pool random number generator
+
+ifeq ($(CONFIG_CRYPTO_RANDOM_POOL),y)
+  CRYPTO_CSRCS += random_pool.c
+endif
+
 endif # CONFIG_CRYPTO
 
 ASRCS = $(CRYPTO_ASRCS)
@@ -97,4 +109,4 @@ distclean: clean
 	$(call DELFILE, Make.dep)
 	$(call DELFILE, .depend)
 
--include Make.dep
+-include Make.dep