firmware-utils: tplink-safeloader: support strings as soft_version

firmware-utils: tplink-safeloader: support strings as soft_version

Some TP-Link routers (C25, C59, C60) contain a version string instead
of a binary structure in the soft_version partition.

Flashing LEDE from the original firmware's GUI, this version string
taken from the soft_ver partition of the firmware image is written to
the router's config partition.

When using tftp recovery to go back to the original Archer C25 firmware,
a version check compares that version to the version of the firmware to
be flashed.

Without proper contents in the config partition, reverting to the
original firmware fails.

Therefore, write the string "soft_ver:1.0.0\n" to that soft_ver
partition.

Signed-off-by: Jan Niehusmann <jan@gondor.com>