bootutil: ecdsa: Fix CC310 for nRF devices

This contains fixes for CC310 for nRF devices using the nrfxlib.
Removed some const for compilation warnings and added `NUM_ECC_BYTES`
define.

Signed-off-by: Sigvart Hovland <sigvart.m@gmail.com>
diff --git a/boot/bootutil/src/image_ec256.c b/boot/bootutil/src/image_ec256.c
index 185624c..f5b8135 100644
--- a/boot/bootutil/src/image_ec256.c
+++ b/boot/bootutil/src/image_ec256.c
@@ -30,6 +30,9 @@
 
 #ifdef MCUBOOT_SIGN_EC256
 /*TODO: remove this after cypress port mbedtls to abstract crypto api */
+#ifdef MCUBOOT_USE_CC310
+#define NUM_ECC_BYTES (256 / 8)
+#endif
 #if defined (MCUBOOT_USE_TINYCRYPT) || defined (MCUBOOT_USE_CC310)
 #include "bootutil/sign_key.h"