bootutil: allow RSA encryption keys without DP/DQ/QP

Allow runtime generation of CRT params when not available in the
embedded private key. Also remove parsing/calculation of CRT
parameters when CRT was disabled in the config (mbedTLS does not
use those in this case).

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/os.c b/boot/zephyr/os.c
index 2d9faae..eaa60b4 100644
--- a/boot/zephyr/os.c
+++ b/boot/zephyr/os.c
@@ -40,7 +40,11 @@
 #if (CONFIG_BOOT_SIGNATURE_TYPE_RSA_LEN == 2048) && !defined(CONFIG_BOOT_ENCRYPT_RSA)
 #define CRYPTO_HEAP_SIZE 6144
 #else
-#define CRYPTO_HEAP_SIZE 10240
+#  if !defined(MBEDTLS_RSA_NO_CRT)
+#  define CRYPTO_HEAP_SIZE 10240
+#  else
+#  define CRYPTO_HEAP_SIZE 16384
+#  endif
 #endif
 
 static unsigned char mempool[CRYPTO_HEAP_SIZE];