Add support for upstream syslinux

To insulate ONIE build from syslinux version changes in different Linux
distro's, we're pulling in a specific `syslinux` version from upstream.

The version selected is the latest from the 4.x series which is what
ONIE has been using till now. The exact version is - `4.07`.

The upstream package already comes bundled with pre-built binaries which
we are using directly (hence no syslinux source build needed).

The `debian-prepare-build-host` target has the `syslinux-common` package
removed as it's no longer needed.
I would put the definition of SYSLINUX_DIR into make/syslinux.make, not here.