util: signer: fix nonzero offset binary image signing bug
util: signer: fix nonzero offset binary image signing bug

When signer code was ported and enhanced to work with binaries (in
addition to hex), a bug was introduced, where the signed image is
considered to always to be based at the bottom of the flash. In fact
the image could be anywhere in the flash, the actual address derived
from the input elf file should be used.

BRANCH=none
BUG=chrome-os-partner:43025
TEST=verified that when enabled, RW image is signed properly
     (build/cr50/RW/ec.RW.flat starts with a signature).

Change-Id: I194bd02f932529165adcbebf4bcbd9020fc261dc
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/311314
Reviewed-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@chromium.org>

coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics