build: fix empty SUBTARGET in json files

Some targets like kirkwood or omap don't use a subtarget which results
in a malformed JSON info file.

Instead of having a valid value like `"target": "ath79/tiny"` for these
targets the value is `"target": "kirkwood/"`.

This patch uses the same if condition to use `generic` if the subtarget
is empty.

Tested for the kirkwood target.

Signed-off-by: Paul Spooren <mail@aparcar.org>