Formalise globbing support for deviceinfo_dtb

@jenneron discovered in https://gitlab.com/postmarketOS/pmaports/-/merge_requests/4250#note_1470930432 that boot-deploy supports globbing for the deviceinfo_dtb variable, which is a very useful feature for devices with a lot of dtbs where listing them all is unwieldy on one line. This is effectively due to an implementation detail (find supporting globbing), which isn't a good thing to rely on in case someone changes the implementation. As such, I think it would be good to formalise that this supports globbing by documenting it and also writing a test that ensures it stays working.