kbuild: allow arch/$(ARCH)/Makefile to override cmd_vmlinux__

The comments said so, so let the code refelct the comment.
First user is um that needs a non-standard link rule.
um patch will be submitted by the UML folks.

Also dropped a bogus comment.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -577,9 +577,8 @@ vmlinux-lds  := arch/$(ARCH)/kernel/vmlinux.lds
 
 # Rule to link vmlinux - also used during CONFIG_KALLSYMS
 # May be overridden by arch/$(ARCH)/Makefile
-# Require first prerequisite to be the lds file
-quiet_cmd_vmlinux__  = LD      $@
-      cmd_vmlinux__  = $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux) -o $@ \
+quiet_cmd_vmlinux__ ?= LD      $@
+      cmd_vmlinux__ ?= $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux) -o $@ \
       -T $(vmlinux-lds) $(vmlinux-init)                          \
       --start-group $(vmlinux-main) --end-group                  \
       $(filter-out $(vmlinux-lds) $(vmlinux-init) $(vmlinux-main) FORCE ,$^)