config.mk: move LDSCRIPT processing to the top-level Makefile

LDSCRIPT is used only from the top-level Makefile and only when the
system is configured so we can move LDSCRIPT and CONFIG_SYS_LDSCRIPT
related logic into the top level Makefile and under configured condition
to avoid errors when building tools from unconfigured tree.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>