I'm using my won package and yes, its format is .apk. I don't want to give our client a whole firmware to install it, at least i'm trying to avoid that, especially i'm not doing any changes to the current features. i'm just adding a new option that can be installed in many firmware versions without forcing our client to use a specific one ;), and after digging in the SDK package i found in the package manager code source that it's looking for something like this "tlt_version" in the check function, but i don't know where i need to add that line of code. i already tested in my makefile like this, but didn't work.
define Package/subMqttMsg
<section, category, title, depends are mentioned here>
tlt_version:RUT9XX_R_00.06.06.1
endef