Makefile.lib: Build all DTS with -@ if OF_LIBFDT_OVERLAY is enabled

The commit 47818e23 ("Makefile.lib: include /__symbols__ in dtb if
SPL_LOAD_FIT_APPLY_OVERLAY is enabled") enables DT building as overlays
based on symbols which depends on SPL. But there is already an option to
apply overlays in full U-Boot too.
And there are platforms which are not using SPL and there is no option to
build DTs with -@ parameter.
That's why change dependency on OF_LIBFDT_OVERLAY which is already symbol
which is selected when SPL_LOAD_FIT_APPLY_OVERLAY is enabled but also
adding support for platforms which don't enable SPL and want to work with
overlays on U-Boot prompt.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>