Makefile: use EXTRAVERSION for -rc? tag

When using SUBLEVEL for the -rc? tag, this results in an ugly version
string like "2013.01.-rc1" - let's use EXTRAVERSION instead to get the
simple "2013.01-rc1"

Signed-off-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Tom Rini <trini@ti.com>

Makefile

@@ -23,8 +23,8 @@
 
 VERSION = 2013
 PATCHLEVEL = 01
-SUBLEVEL = -rc1
-EXTRAVERSION =
+SUBLEVEL =
+EXTRAVERSION = -rc1
 ifneq "$(SUBLEVEL)" ""
 U_BOOT_VERSION = $(VERSION).$(PATCHLEVEL).$(SUBLEVEL)$(EXTRAVERSION)
 else