bootutil: zephyr: Fix not linking with mbedtls when needed

This fixes a build issue when building mcuboot for zephyr with RSA
image encryption support enabled using mbedtls.

Signed-off-by: Jamie McCrae <jamie.mccrae@lairdconnect.com>
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>