BL2: Define bootutil_key_cnt for ECDSA keys

Compiling with the options:
- MCUBOOT_SIGNATURE_TYPE=EC-P256
- MCUBOOT_BUILTIN_KEY=OFF
- MCUBOOT_HW_KEY=OFF

causes the following linker errors:

/opt/zephyr-sdk-1.0.1/gnu/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/14.3.0/../../../../arm-zephyr-eabi/bin/ld: bl2/ext/mcuboot/bootutil/libbootutil.a(image_validate.o): in function `bootutil_img_validate':
/home/user/west_workspace/bootloader/mcuboot/boot/bootutil/src/image_validate.c:601:(.text.bootutil_img_validate+0x1e8): undefined reference to `bootutil_key_cnt'
/opt/zephyr-sdk-1.0.1/gnu/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/14.3.0/../../../../arm-zephyr-eabi/bin/ld: bl2/ext/mcuboot/bootutil/libbootutil.a(bootutil_find_key.o): in function `bootutil_find_key':
/home/user/west_workspace/bootloader/mcuboot/boot/bootutil/src/bootutil_find_key.c:69:(.text.bootutil_find_key+0x7c): undefined reference to `bootutil_key_cnt'

Fix this by defining bootutil_key_cnt in keys.c for ECDSA keys.

Change-Id: Icea5b17986fa86e262c8c14e3786524c125c70c3
Signed-off-by: Tim Pambor <tim.pambor@codewrights.de>

Author: tpambor

bl2/ext/mcuboot/keys.c

@@ -397,5 +397,6 @@ const struct bootutil_key bootutil_keys[] = {
     },
 #endif /* MCUBOOT_IMAGE_NUMBER > 3 */
 };
+const int bootutil_key_cnt = MCUBOOT_IMAGE_NUMBER;
 #endif /* MCUBOOT_SIGN_RSA */
 #endif /* HAVE_KEYS */