kbuild: Move linker sciript check to prepare1

Same as the previous commit.
Move sanity check to prepare1 target to avoid nasty troubles.

Before this commit, LDSCRIPT existence was not checked
when it was specified by CONFIG_SYS_LDSCRIPT.
Now LDSCRIPT existence is checked for all boards.

$(wildcard $(LDSCRIPT)) must point to the linker scripts
with absolute path.
Otherwise, make will terminate with a false error
on out-of-tree build.

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