config: expose path to device package

Expose the path to the selected device aports package, this makes it much easier for tools like my installer to easily locate the deviceinfo file as I can go from something like this (which only works for device in a single category):

DEVICEINFO_PATH="$PMBWORK/cache_git/pmaports/device/community/device-$PMBDEVICE/deviceinfo"

To this

DEVICEINFO_PATH="$(pmbootstrap config device_aports_path)/deviceinfo"

I'm open to making it a relative path instead.