device-generic-x86_64: create vendor-specific subpackages

From the discussion in !4554 (merged), this device package should have vendor-specific (e.g. *-intel and *-amd) subpackages to install vendor-specific packages (e.g., microcode, vulkan loaders, sof).

_pmb_select can be used when building images with pmbootstrap, so that users can select the vendor they prefer. Pre-built images for this device should include all vendor packages so that it's compatible with devices from either vendor.