Fix an issue in assert-max-image-size.

When building the recovery image on marlin (m bootimage):
----- Making recovery image ------
Copying baseline ramdisk...
Modifying ramdisk contents...
/bin/bash: line 0: [: 0x02000000: integer expression expected
----- Made recovery image: out/target/product/marlin/boot.img --------

Because if -gt doesn't like hexadecimal numbers. Fix the issue by doing
a dummy arithmetic to get integer $$maxsize.

Test: `m bootimage` with aosp_marlin-userdebug. Also change the
      BOARD_BOOTIMAGE_PARTITION_SIZE closer to the actual size to
      trigger the different size check paths.
Change-Id: I75c7727664d7255b9c18f57ae38076ff90b8a957