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>