This new mechanism can be used to make wpa_supplicant using external
storage (e.g., key store in the operating system) for passwords,
passphrases, and PSKs. This commit is only adding the framework part
needed to support this, i.e., no actual configuration parameter can
yet use this new mechanism. In addition, only a simple test backend
is added to allow developer testing of the functionality.
Signed-hostap: Jouni Malinen <j@w1.fi>
