tplink-safeloader: add QNEW image detection

tplink-safeloader: add QNEW image detection

An incompatible image type is now used, e.g. for the TL-WPA8631P v4,
which has a header containing 0x3C extra bytes. This image type can be
identified by the first four bytes of the image header being "?NEW".

Only detection is implemented at this moment, as the full header format
is not yet understood, and the preamble checksum can no longer be
verified using the current md5sum salt. Devices still appear to accept
images in the "old" format, so image generation is not required at this
moment.

Cc: Andreas Böhler <dev@aboehler.at>
Signed-off-by: Sander Vanheule <sander@svanheule.net>