tools: checkstack.pl from Linux added to tools

Added from Linux - commit 62aa2b537c6f5957afd98e29f96897419ed5ebab

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/Makefile b/Makefile
index 33136ff..1446131 100644
--- a/Makefile
+++ b/Makefile
@@ -523,6 +523,11 @@
 FIND := find
 FINDFLAGS := -L
 
+checkstack:
+		$(CROSS_COMPILE)objdump -d $(obj)u-boot \
+			`$(FIND) $(obj) -name u-boot-spl -print` | \
+			perl $(src)tools/checkstack.pl $(ARCH)
+
 tags ctags:
 		ctags -w -o $(obj)ctags `$(FIND) $(FINDFLAGS) $(TAG_SUBDIRS) \
 						-name '*.[chS]' -print`