scripts/make-index-json: ensure that manifest output is in opkg format

scripts/make-index-json: ensure that manifest output is in opkg format

Certain existing tooling, such as 'package-metadata.pl', are written
to accept the output of 'opkg list' with package manifest delimited
by '-'.  The 'make-index-json.py --manifest' output was emulating
the 'apk list --manifest' format without the delimiting dash,
thus breaking these legacy tools.

We fix this by adding the dash to the manifest output, which allows
all existing tooling to process the output irrespective of whether
the build system uses opkg or apk.

Signed-off-by: Eric Fahlgren <ericfahlgren@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20094
Signed-off-by: Robert Marko <robimarko@gmail.com>