fs_mgr_verity: support updating actual block device path in verity table

The value of PRODUCT_{SYSTEM,VENDOR}_VERITY_PARTITION will be included
in the verity metadata of an image. For example:

    PRODUCT_VENDOR_VERITY_PARTITION := /dev/block/platform/soc.0/f9824900.sdhci/by-name/vendor

This makes the image cannot be shared for devices with different by-name
prefix:
    /dev/block/platform/soc.0/f9824900.sdhci/by-name/vendor
    /dev/block/platform/soc.0/7824900.sdhci/by-name/vendor

This change supports using a generic PRODUCT_{SYSTEM,VENDOR}_VERITY_PARTITION to
build image and replace it with the actual path at run time:

    PRODUCT_VENDOR_VERITY_PARTITION := /dev/block/bootdevice/by-name/vendor ==>
    /dev/block/platform/soc.0/7824900.sdhci/by-name/vendor

Bug: 62171170
Test: Boot sailfish with PRODUCT_VENDOR_VERITY_PARTITION := /dev/block/bootdevice/by-name/vendor
Change-Id: I2789d2a025e80a628a1b4e7419818065c209fb7f