linux: fix build with release candidates

Fixes *** Recursive variable `LINUX_SITE' references itself

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

linux/linux.mk

@@ -23,7 +23,7 @@ LINUX_VERSION_MINOR = $(word 2,$(subst ., ,$(subst -, ,$(LINUX_VERSION))))
 LINUX_SITE = $(BR2_KERNEL_MIRROR)/linux/kernel/v$(LINUX_VERSION_MAJOR).$(LINUX_VERSION_MINOR)/
 # release candidates are in testing/ subdir
 ifneq ($(findstring -rc,$(LINUX_VERSION)),)
-LINUX_SITE = $(LINUX_SITE)testing/
+LINUX_SITE := $(LINUX_SITE)testing/
 endif # -rc
 endif