deviceinfo_bootimg_dtb_second needs update for new DTBs path

https://gitlab.postmarketos.org/postmarketos/boot-deploy/-/blob/d6d77fe0e6a03c3e718f8b99b0d2db49a18dd485/boot-deploy-functions.sh#L400-417 currently only checks the old DTBs path in /usr/share/dtbs, which means it fails now after all the kernels in pmaports have been changed to install to /boot/dtbs/. See e.g. https://builds.sr.ht/~postmarketos/job/861218

It affects all devices using deviceinfo_bootimg_dtb_second, e.g. arrow-db410c or samsung-skomer in postmarketOS.