kbuild: asm symlink support for arch/$ARCH/include

Adjust the asm symlink support so we do not create the
symlink unless really needed.
We check the precense of include/asm-$ARCH by checking
for the system.h file. We may end up with a stale directory
so it is not enough to check if the directory is present.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Kbuild b/Kbuild
index e750e9c..f056b4f 100644
--- a/Kbuild
+++ b/Kbuild
@@ -43,7 +43,7 @@
 # 2) Generate asm-offsets.h
 #
 
-offsets-file := include/asm-$(SRCARCH)/asm-offsets.h
+offsets-file := include/asm/asm-offsets.h
 
 always  += $(offsets-file)
 targets += $(offsets-file)
@@ -81,7 +81,6 @@
 	$(call if_changed_dep,cc_s_c)
 
 $(obj)/$(offsets-file): arch/$(SRCARCH)/kernel/asm-offsets.s Kbuild
-	$(Q)mkdir -p $(dir $@)
 	$(call cmd,offsets)
 
 #####