HS 2.0: Add a command to retrieve icon with in-memory storage

HS 2.0: Add a command to retrieve icon with in-memory storage

This adds a new command based Hotspot 2.0 icon retrieval option.

In short, here is the new command sequence:
1. REQ_HS20_ICON <bssid> <file-name>
2. event: RX-HS20-ICON <bssid> <file-name> <size>
3. GET_HS20_ICON <bssid> <file-name> <offset> <size>
   (if needed, repeat with larger offset values until full icon is
   fetched)
5. DEL_HS20_ICON <bssid> <file-name>

REQ_HS20_ICON is a new command that is analogous to HS20_ICON_REQUEST
with the slight difference that an entry to store the icon in memory is
prepared. The RX-HS20-ICON event has been augmented with BSSID,
file-name and size, and GET_HS20_ICON is used to retrieve a chunk of up
to <size> bytes of icon data at offset <offset>. Each chunk is returned
as a base64 encoded fragment, preceded by "HS20-ICON-DATA", BSSID, and
file-name as well as the starting offset of the data.

If there is no entry prepared for the icon when the ANQP result comes
back, hs20_process_icon_binary_file falls back to legacy behavior.

Finally the DEL_HS20_ICON command deletes (all) icons associated with
BSSID and file-name (there could be several if retries are used and they
have different dialog tokens).

Signed-off-by: Jan Nordqvist <jannq@google.com>