Fix Windows build: add default_CSPRNG and improve build.sh error handling

- Add Windows implementation of default_CSPRNG() using CryptGenRandom
- Add fallback stub for platforms without CSPRNG support
- Fix build.sh to properly detect build failures (use PIPESTATUS)
- Show proper error message instead of "Build completed successfully" on failure

tinycrypt/lib/source/ecc_platform_specific.c

@@ -101,5 +101,36 @@ int default_CSPRNG(uint8_t *dest, unsigned int size) {
   return 1;
 }
 
+#elif defined(_WIN32)
+/* Windows implementation using CryptGenRandom */
+#include <windows.h>
+#include <wincrypt.h>
+
+int default_CSPRNG(uint8_t *dest, unsigned int size) {
+  /* input sanity check: */
+  if (dest == (uint8_t *) 0 || (size <= 0))
+    return 0;
+
+  HCRYPTPROV hProvider = 0;
+  
+  if (!CryptAcquireContext(&hProvider, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT | CRYPT_SILENT)) {
+    return 0;
+  }
+  
+  BOOL result = CryptGenRandom(hProvider, size, dest);
+  
+  CryptReleaseContext(hProvider, 0);
+  
+  return result ? 1 : 0;
+}
+
+#else
+/* No CSPRNG available - return error */
+int default_CSPRNG(uint8_t *dest, unsigned int size) {
+  (void)dest;
+  (void)size;
+  return 0;
+}
+
 #endif /* platform */