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
Change-Id: I0594814fe19ffc54dff41ec2464e2e3a802b9419