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>
diff --git a/include/configs/MPC8572DS.h b/include/configs/MPC8572DS.h
index f457719..7b63945 100644
--- a/include/configs/MPC8572DS.h
+++ b/include/configs/MPC8572DS.h
@@ -24,7 +24,7 @@
 #define CONFIG_SYS_TEXT_BASE_SPL 0xfff00000
 #define CONFIG_SYS_MONITOR_BASE	CONFIG_SYS_TEXT_BASE_SPL /* start of monitor */
 #else
-#define CONFIG_SYS_LDSCRIPT $(TOPDIR)/$(CPUDIR)/u-boot-nand.lds
+#define CONFIG_SYS_LDSCRIPT $(CPUDIR)/u-boot-nand.lds
 #define CONFIG_SYS_TEXT_BASE	0xf8f82000
 #endif /* CONFIG_NAND_SPL */
 #endif