project-lakitu: restrict syslinux and i7z for amd64 only

Syslinux is made only for x86 compatible architectures, and i7z is
obviously only for Intel's architecture.

We don't use syslinux in COS anymore. However, the image building
scripts in src/scripts depend on syslinux, when the architecture is
x86 compatible. Hence we still need to keep syslinux for amd64.

BUG=b/170716822
TEST=No effect on existing builds. Did 'cuskyctl builds submit lakitu'
RELEASE_NOTE=None

Change-Id: I16bc80d1ed6369bcd8243a844a1c305df83de5d4
4 files changed