tqftpserv: Implement path translation

The tftp clients in the modem requests files from either /readwrite or
/readonly/firmware/image, but as the wlanmdsp.mbn file requested is
signed with a vendor specific key these paths needs to be translated.

/readwrite is translated to /tmp/tqftpserv for now and the
/readonly/firmware/image is translated to a search for the requested
file alongside any of the remoteproc firmware files. The result is that
the wlanmdsp.mbn can be stored in linux-firmware alongside it's
device/vendor specific modem firmware.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>