Add option include a pvmfw partition

Protected KVM relies on protected VM firmware (pvmfw) to bootstrap the
trust in protected VMs. This firmware is AVB protected and stored in the
pvmfw partition.

Test: build a target with BOARD_PREBUILT_PVMFWIMAGE set, saw the image
      included in the product files and used avbtool to check the footer
      and vbmeta.img details.
Bug: 171280178
Change-Id: I4d6a1819f307c77ca60c0dd3dff39ee28cece4f6