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>
diff --git a/boot/bootutil/zephyr/CMakeLists.txt b/boot/bootutil/zephyr/CMakeLists.txt
index d66751b..efdbf60 100644
--- a/boot/bootutil/zephyr/CMakeLists.txt
+++ b/boot/bootutil/zephyr/CMakeLists.txt
@@ -48,4 +48,8 @@
   ../../../ext/tinycrypt/lib/include
 )
 endif()
+
+if(CONFIG_BOOT_USE_MBEDTLS)
+  zephyr_link_libraries(mbedTLS)
+endif()
 endif()