EXT PW: Add support for password parameter from external storage

EXT PW: Add support for password parameter from external storage

This allows the password parameter for EAP methods to be fetched
from an external storage.

Following example can be used for developer testing:

ext_password_backend=test:pw1=password|pw2=testing

network={
    key_mgmt=WPA-EAP
    eap=TTLS
    identity="user"
    password=ext:pw1
    ca_cert="ca.pem"
    phase2="auth=PAP"
}

Signed-hostap: Jouni Malinen <j@w1.fi>