Revert "Remove pkcs11-helper option"

This reverts commit d832f187f756079552601867348d924582bf65de.

Conflicts:
* CMakeLists.txt:
  * USE_PKCS11_HELPER_LIBRARY: there has been a change immediately before
    where it was removed. Just re-add what was removed.
* tests/CMakeLists.txt:
  * USE_PKCS11_HELPER_LIBRARY: there has been a change immediately before
    where it was removed. Just re-add what was removed.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4d40eea..9c6fb5c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -8,6 +8,7 @@
 # Set the project root directory.
 set(MBEDTLS_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 
+option(USE_PKCS11_HELPER_LIBRARY "Build mbed TLS with the pkcs11-helper library." OFF)
 option(ENABLE_ZLIB_SUPPORT "Build mbed TLS with zlib library." OFF)
 
 option(ENABLE_PROGRAMS "Build mbed TLS programs." ON)