Makefile: refactor ubootrelease target

Instead of duplicating the contents of the filechk_uboot.release
variable, use it directly.

This is preparation for the next patch which will modify
filechk_uboot.release, and reflects what the linux kernel does
nowadays:

kernelrelease:
	@$(filechk_kernel.release)

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
diff --git a/Makefile b/Makefile
index 44deb33..efaf24e 100644
--- a/Makefile
+++ b/Makefile
@@ -2426,7 +2426,7 @@
 	$(PERL) $(src)/scripts/checkstack.pl $(ARCH)
 
 ubootrelease:
-	@echo "$(UBOOTVERSION)$$($(CONFIG_SHELL) $(srctree)/scripts/setlocalversion $(srctree))"
+	@$(filechk_uboot.release)
 
 ubootversion:
 	@echo $(UBOOTVERSION)