build_image: log emerge commands

It helps to see the exact emerge command when installing packages
into the rootfs images.  This adds ~5 lines to build_image which
should be nothing.

BUG=None
TEST=`./build_image --board=kumo test` produces some nice log lines

Change-Id: I2c781d51d07bb48772528cbbb13b4eb80fde5712
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/1928571
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed