ldscript_fallback_cbs.lb and 4G files

[Maciej Pijanka]

Hello

Sorry for troubles caused by my ldscript patch that triggered slightly
misbehaviour resulting in 4GB image files in abuild. I didn't noticed
such behaviour on my machine and thats why i send it to ML.

I created separated from ld way to reuse space in top 64k not used by bootblock
patch in attachment. Only missing thing in this patch would be option to enable
firing script that calculates size that can be claimed and adjusting gapsize
so it might be disabled by default.

Signed-off: Maciej Pijanka 
(or should i add Blames-To: also?)

best regards
Maciej