futility: dump_fmap: Fix gap calculation

When fmap_base is non-zero, `futility dump_fmap -H` shows the addresses
relative to fmap_base, except that the "-entire flash-" line contains
absolute addresses. The problem also results in incorrect calculation of
the gap sizes.

Fix the problem by always showing the relative addresses.

Current output:

 name          start       end         size
 -entire flash-  10090000    10110000    00080000
                   00070000    10110000    100a0000  // gap in -entire flash-

Fixed output:

 name          start       end         size  // address relative to base=0x1009000
 -entire flash-  00000000    00080000    00080000
                   00070000    10110000    100a0000  // gap in -entire flash-

BUG=none
TEST=make futil -j
TEST=futility dump_fmap -H ec.bin
BRANCH=none

Change-Id: Ifc164e7ff50bc3688e53518a280d309094f4152d
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/6342833
Reviewed-by: Julius Werner <jwerner@chromium.org>