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.
The inclusion of the syslinux.make file needs to be conditional as it makes no sense for PowerPC platforms.  We need something like:
```makefile
ifeq ($(SYSLINUX_ENABLE),yes))
    include make/syslinux.make
endif
```

Following that we could put SYSLINUX_ENABLE=yes into build-config/arch/x86_64.make

Then we only pick it up on x86_64 platforms.

We will need to test $(SYSLINUX_ENABLE) in a few more places, see below.