Makefile: delete a make rule of $(LDSCRIPT)

$(LDSCRIPT) is a source file, not a generated file.
We do not need a make rule of $(LDSCRIPT).

And one more trivial fix:
$(obj)/u-boot should not dierectly depend on $(LDSCRIPTS).

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>