When the flash base address is not zero (e.g. STM32L chip), the current
image index is wrongly computed.
Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=None
TEST=check it compiles for all boards, run on Discovery board and see
proper value.
Change-Id: I06f5508cdffce6d754bd93e870d64087d299c9c7