It was originally hard-coded at 0x200 which is 512 sectors. This
only works for the H100 and H120. The larger ROM of the H300 is 1024
sectors in size. In either case the bootloader starts 16 sectors before
the end of the ROM so rely on this fact to correctly calculate where to
stop the bootloader erasure.
Change-Id: Iec4112ebf24379f80a7bf1363035e005c434907e