Refactor vendor_boot v4 build logic and add more checks

Add more checks around BOARD_BOOT_HEADER_VERSION in board_config.mk.

Refactor generation logic of vendor_boot ramdisk fragments.
Consolidate initialization and validation check to its own section.
Adjust some variable names and initialization sequence so the follow-up
change can land more cleanly.
Rename variable name "dir" so that it don't collide with the Makefile
function "$(dir ...)".

Bug: 183395459
Test: Presubmit; Change should be no-op with respect to build artifacts.
Test: Modify BoardConfig of CF and m dist. Verify the vendor_boot.img
  with unpack_bootimg.
Change-Id: I8785c40dd9f87f3797a56ada93e65939d27d0e9b