kbuild: Avoid "expr length" in Makefile

From:  Martin Schaffner <maschaffner@gmx.ch>

Makefile

@@ -662,7 +662,7 @@ include/config/MARKER: include/linux/autoconf.h
 uts_len := 64
 
 define filechk_version.h
-	if expr length "$(KERNELRELEASE)" \> $(uts_len) >/dev/null ; then \
+	if ((`echo -n "$(KERNELRELEASE)" | wc -c ` > $(uts_len))); then \
 	  echo '"$(KERNELRELEASE)" exceeds $(uts_len) characters' >&2; \
 	  exit 1; \
 	fi; \