Custom BSPs use an alternate method to specify dependencies. The advantage is that the specification of the dependencies is self-contained within the custom BSP itself.
This method is accomplished by having files with the extension mtbx for each dependency in the deps directory inside the custom BSP. An mtbx file has the exact same content at an mtb file with a different file extension.
The process of getting dependencies from mtbx files only searches in the target BSP
for the application, so if your application contains multiple custom BSPs, only the
dependencies for the active target (as specified in the TARGET variable in
the Makefile or on the command line) will be included in the application.
The mtbx files are automatically created for you, so you typically don't need to do anything with them unless you want to add or remove dependencies from your custom BSP. You can use the BSP Assistant to do this; for details, refer to the BSP Assistant user guide.
During make getlibs, mtbx files are treated just like indirect
dependencies that are specified in a manifest file. That is, the process will copy each
mtbx file to the application's libs directory and will lock the version in
the assetlocks.json file in the deps directory. The libraries are then pulled
down according to the information contained in the mtb files.