linux: fix missing default packages in profiles.json

linux: fix missing default packages in profiles.json

Robert reported, that in firmware images generated by ASU, there is
`apk` package manager missing after the commit 44598c233dd9 ("build:
remove broken dependency of metadata on toplevel .config variables").

That is happening, because apk got removed from `default_packages` list in
`profiles.json`, which is being generated by `json_overview_image_info` Make
target, which uses `scripts/json_overview_image_info.py` helper script,
which gets the information from `DEFAULT_PACKAGES` Make variable.

So lets fix it by providing `DEFAULT_PACKAGES` variable when its needed.

The reason why we didn't added those packages as a dependency to
base-files like any other packages, was to allow disabling them (in
order to save space).

Fixes: #16969
Fixes: openwrt/asu/issues/1084
Fixes: 44598c233dd9 ("build: remove broken dependency of metadata on toplevel .config variables")
Reported-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Link: https://github.com/openwrt/openwrt/pull/16986
Signed-off-by: Robert Marko <robimarko@gmail.com>