Introduce BOARD_PVMFWIMAGE_PARTITION_SIZE

Rename the BOARD_PVMFWIMG_PARTITION_SIZE variable to follow the format
used by all other partitions: BOARD_$(name)IMAGE_PARTITION_SIZE. Note
that all boards using that variable should have been updated to also
define the new version, by now.

Define the new variable as board read-only, as done for other
partitions.

Bug: 199717422
Test: m ${ANDROID_PRODUCT_OUT}/pvmfw.img
Change-Id: I5664c903db6388458e906e996115b695220932ca