Support journal size board config variables.

A device can set up BOARD_SYSTEMIMAGE_JOURNAL_SIZE (number of journal blocks)
in its BoardConfig.mk.
To disable journaling, set "BOARD_SYSTEMIMAGE_JOURNAL_SIZE := 0".

BOARD_VENDORIMAGE_JOURNAL_SIZE and BOARD_OEMIMAGE_JOURNAL_SIZE work in
similar way.

Bug: 18430740

(cherry picked from commit f3b86357)

Change-Id: Icc6c7e1b3be25b8e21b6a737492d944428e4a35a