pack_firmware: Add message when skipping optional sections
pack_firmware: Add message when skipping optional sections

This helps understand what is in the image, if -v3 is used.

BUG=none
TEST=manual
$ cros_bundle_firmware -u u-boot.bin -b daisy -d exynos5250-snow
  -I arch/arm/dts -I cros/dts -O out  -v3 -s

See the messages:
Section 'FMAP' is not required, skipping
Section 'RO_VPD' is not required, skipping
Section 'RO_FRID' is not required, skipping
Section 'GBB' is not required, skipping
...

Change-Id: I439b45278fc7eceb2c2c4d414fe35a9ca1699274
Reviewed-on: https://gerrit.chromium.org/gerrit/27981
Reviewed-by: Tom Wai-Hong Tam <04b587fdf5845741a0c5a8b9cd59ca72d73ef8fc@chromium.org>
Commit-Ready: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Tested-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

coreos/dev-util,coreos/dev-util,mgireesh05/dev-util,coreos/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,marineam/coreos-dev-util,mgireesh05/dev-util