Stop assuming that pvmfw.img can only be pre-built

Introduce the BOARD_USES_PVMFWIMAGE variable, similarly to all other
partitions, and use it where appropriate (in particular, where the
soon-to-be incorrect assumption that pvmfw.img could only be a pre-built
board image was made).

Bug: 199717422
Test: m ${ANDROID_PRODUCT_OUT}/pvmfw.img # with TARGET_PKVM_ENABLED=true
Change-Id: I8f4faa78c741d29b473303b521834387dbd48cd1