Add --arch flag to pack mode of vbutil_kernel

When --arch flag is not x86, x86-only operations in pack mode are
turned off so that we can reuse vbutil_kernel to generate kernel partition
images for other targets, such as arm.

See CL:6538014 for its application.

BUG=chromium-os:3790
TEST=Run "emerge vboot_reference" successfully

Review URL: http://codereview.chromium.org/6538015

Change-Id: If45cf092d1ecc762fad6fda1aa57d23e26a7e47a
1 file changed