...to *not* error out if there's no gfxboot at all.

This change is done to reduce ambiguity in some cases;
the previous intention has been to ease navigation when
staying in a particular directory, now it's been changed
in favour of convenient toplevel `git grep' in fact.

Both variants have their pros and cons, I just find myself
leaning to this one by now hence the commit.  Feel free to
provide constructive criticism :)

Some path-related bitrot has also been fixed while at that.

alterator-netinst currently relies on "default"
being specified explicitly; it's wrong and it should
cope with the first "label" clause as well but we're
better off being strict to this script, not that one.

This commit should be no-op regarding syslinux itself.

This one was suggested by enp@ for industrial use where
some extra protection for the boot process might be quite
desirable.

If no syslinux ui was specified (the stock configuration paths
ensure there is one) or if it was set to "none" explicitly,
then there's no boot: prompt (let alone any menu).

If there's a need to ensure that the boot process is not
interruptable by Ctrl/Shift/Caps Lock/Scroll Lock.

Avoid extra files where a variable would suffice; and use
the more specific "syslinux.list" rather than a "files.list".

Fixed up the remnants of the early style mix
to correspond to the proposed doc/style.txt;
the rationale being that

	if [ ... ]; then
		...
		...
	fi

is the more readable construct among itself,

	if test ...; then
		...
		...
	fi

and

	[ ... ] && {
		...
		...
	}

due to the condition being more distinguishable
when bracketed and the body more apparent as the
one inside "if" and not any other block; the less
obvious difference is that the final construct of
the latter form is prone to the whole script exit
status being non-zero if the condition isn't met.

Implementation based on m-p-d::profiles/scripts.d/03-syslinux
(but heavily modified for m-p of course)

Renamed server-light.iso into server-ovz.iso to avoid brand dilution
and confusion (rider@'s server-light rather favours kvm, anyways).

Introduced KDEFAULT: a reliable default kernel chooser knob
since apt's regex ordering proved pretty unreliable.
Spelling things explicitly is better anyways.

SYSLINUX related features undergone pretty major rewrite
(that includes syslinux, hdt and memtest).

The problem to tackle was features.in/syslinux/generate.mk
assuming syslinux and pciids available in build *host* system;
this well might not be the case (or worse yet, those can be
just different).  So now we're a bit less elegant and a bit
more enterprise, stuffing things into chroot and working there.

Bunch of other fixes along the road, including ; to name a few:
- fixed memtest entry (overlooked while renaming SYSLINUX_ITEMS)
- new and shiny doc/CodingStyle
- gfxboot, stage1 target chain, hdt tweaks
- distro.mk rehashed
- README++
- TODO: dropped (integer overflow anyways)
  + actually moved off-tree to reduce commit spam
- s,\.config\.mk,distcfg.mk,g
- doc/profiles.mk.sample: sample ~/.mkimage/profiles.mk
- ...and assorted fixups/additions

Sorry for convoluted commit, this would have been pretty hard to
rework into some really readable shape (and you might be interested
in the original repo's history horrors then, anyways).

Also included:
- reworked syslinux feature inclusion
- syslinux: s/textprompt/prompt/
- README++