arm64: vdso: fix and clean-up Makefile

- $(call if_changed,...) must have FORCE as a prerequisite

- vdso.lds is a generated file, so it should be prefixed with
  $(obj)/ instead of $(src)/.

- cmd_vdsosym is a one-liner rule, so the assignment with '='
  is simpler.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/vdso/Makefile

@@ -31,7 +31,7 @@ CPPFLAGS_vdso.lds += -P -C -U$(ARCH)
 $(obj)/vdso.o : $(obj)/vdso.so
 
 # Link rule for the .so file, .lds has to be first
-$(obj)/vdso.so.dbg: $(src)/vdso.lds $(obj-vdso)
+$(obj)/vdso.so.dbg: $(obj)/vdso.lds $(obj-vdso) FORCE
 	$(call if_changed,vdsold)
 
 # Strip rule for the .so file
@@ -42,9 +42,7 @@ $(obj)/%.so: $(obj)/%.so.dbg FORCE
 # Generate VDSO offsets using helper script
 gen-vdsosym := $(srctree)/$(src)/gen_vdso_offsets.sh
 quiet_cmd_vdsosym = VDSOSYM $@
-define cmd_vdsosym
-	$(NM) $< | $(gen-vdsosym) | LC_ALL=C sort > $@
-endef
+      cmd_vdsosym = $(NM) $< | $(gen-vdsosym) | LC_ALL=C sort > $@
 
 include/generated/vdso-offsets.h: $(obj)/vdso.so.dbg FORCE
 	$(call if_changed,vdsosym)
@@ -55,7 +53,7 @@ $(obj-vdso): %.o: %.S FORCE
 
 # Actual build commands
 quiet_cmd_vdsold = VDSOL   $@
-      cmd_vdsold = $(CC) $(c_flags) -Wl,-n -Wl,-T $^ -o $@
+      cmd_vdsold = $(CC) $(c_flags) -Wl,-n -Wl,-T $(real-prereqs) -o $@
 quiet_cmd_vdsoas = VDSOA   $@
       cmd_vdsoas = $(CC) $(a_flags) -c -o $@ $<