Reapply "pkgbase: Create a FreeBSD-dtb package"

This reverts commit 9bbe06b004e2cadb55a3792c77991ee640bf0f52.

Before that dtbs where included in each kernel packages which prevents
us to install multiple kernels.

Fix mtree-to-plist.awk to only generate a dtb.plist when we create
the kernel one (because dtb are installed during make installkernel).

Sponsored by: Beckhoff Automation GmbH & Co. KG