wifi-scripts: restore priv_key/priv_key_pwd as config aliases

wifi-scripts: restore priv_key/priv_key_pwd as config aliases

The shell config generator read the client private key from the UCI
options priv_key / priv_key_pwd (and priv_key2 / priv_key2_pwd for the
inner tunnel). The ucode generator was switched to private_key /
private_key_passwd (matching the wpa_supplicant field names), and LuCI
was updated accordingly, but existing configurations still carry the old
option names.

Such configs silently lose their private key: for an EAP-TLS client this
leaves wpa_supplicant without a client key and authentication fails after
upgrading from 24.10.

The schema already lists priv_key / priv_key_pwd, but as plain strings,
so validate() never migrates them. Declare them (and the missing
priv_key2 / priv_key2_pwd) as aliases of the private_key* options so the
old names keep working.

Fixes: https://github.com/openwrt/openwrt/issues/22599
Fixes: 218f3884d2fb ("wifi-scripts: add ucode based scripts")
Assisted-by: Claude:claude-opus-4-8
Link: https://github.com/openwrt/openwrt/pull/24088
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>