lantiq: fritz_cal_extract with reverse option for AVM FritzBox 7430

lantiq: fritz_cal_extract with reverse option for AVM FritzBox 7430

This implementation of fritz_cal_extract can also retrieve firmware
data stored in reverse byte order, as found in the AVM 7430 device.

This is done by intermediate storage in a buffer presumably large enough
to hold the complete data set. Currently, this buffer size is 128kB + 1kB
(some extra space for skipped data).

In the usual case of "forward" data, this implementation should behave
like the original implementation in all common cases. limit [-l] will
determine the amount of data read and size of buffer allocated.

However, if you are reading reversed data or didn't set a limit, the buffer
may be too small to hold all data. In this case, you can choose a higher
limit [-l] to enforce a sufficient buffer size.

Signed-off-by: Dustin Gathmann <dzsoftware@posteo.org>
Link: https://github.com/openwrt/openwrt/pull/15501
Signed-off-by: Robert Marko <robimarko@gmail.com>