External persistent storage for PMKSA cache entries

External persistent storage for PMKSA cache entries

This adds new wpa_supplicant control interface commands PMKSA_GET and
PMKSA_ADD that can be used to store PMKSA cache entries in an external
persistent storage when terminating a wpa_supplicant process and then
restore those entries when starting a new process. The previously added
PMKSA-CACHE-ADDED/REMOVED events can be used to help in synchronizing
the external storage with the memory-only volatile storage within
wpa_supplicant.

"PMKSA_GET <network_id>" fetches all stored PMKSA cache entries bound to
a specific network profile. The network_id of the current profile is
available with the STATUS command (id=<network_id). In addition, the
network_id is included in the PMKSA-CACHE-ADDED/REMOVED events. The
output of the PMKSA_GET command uses the following format:

<BSSID> <PMKID> <PMK> <reauth_time in seconds> <expiration in seconds>
<akmp> <opportunistic>

For example:

02:00:00:00:03:00 113b8b5dc8eda16594e8274df4caa3d4 355e98681d09e0b69d3a342f96998aa765d10c4459ac592459b5efc6b563eff6 30240 43200 1 0
02:00:00:00:04:00 bbdac8607aaaac28e16aacc9152ffe23 e3dd6adc390e685985e5f40e6fe72df846a0acadc59ba15c208d9cb41732a663 30240 43200 1 0

The PMKSA_GET command uses the following format:

<network_id> <BSSID> <PMKID> <PMK> <reauth_time in seconds> <expiration
in seconds> <akmp> <opportunistic>

(i.e., "PMKSA_ADD <network_id> " prefix followed by a line of PMKSA_GET
output data; however, the reauth_time and expiration values need to be
updated by decrementing them by number of seconds between the PMKSA_GET
and PMKSA_ADD commands)

For example:

PMKSA_ADD 0 02:00:00:00:03:00 113b8b5dc8eda16594e8274df4caa3d4 355e98681d09e0b69d3a342f96998aa765d10c4459ac592459b5efc6b563eff6 30140 43100 1 0
PMKSA_ADD 0 02:00:00:00:04:00 bbdac8607aaaac28e16aacc9152ffe23 e3dd6adc390e685985e5f40e6fe72df846a0acadc59ba15c208d9cb41732a663 30140 43100 1 0

This functionality is disabled be default and can be enabled with
CONFIG_PMKSA_CACHE_EXTERNAL=y build configuration option. It should be
noted that this allows any process that has access to the wpa_supplicant
control interface to use PMKSA_ADD command to fetch keying material
(PMK), so this is for environments in which the control interface access
is restricted.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>