target.mk: init default configs if they are missing in the .config

target.mk: init default configs if they are missing in the .config

The config options that are enabled by default and where other default
packages depends on should not only be set if there is no .config file,
but also if the .config exists but the config option (e.g.
CONFIG_SECCOMP) is missing in the file.

This is relevant, for example, if you are working with .config templates
and then want to complete the configuration using make defconfig.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>