selftests/powerpc: Fix standalone powerpc build

The changes to enable building with a separate output directory, in
commit a8ba798bc8ec ("selftests: enable O and KBUILD_OUTPUT") broke
building the powerpc selftests on their own, eg:

 $ cd tools/testing/selftests/powerpc; make

It was partially fixed in commit e53aff45c490 ("selftests: lib.mk Fix
individual test builds"), which defined OUTPUT for standalone tests. But
that only defines OUTPUT within the Makefile, the value is not exported
so sub-shells can't see it. We could export OUTPUT, but it's actually
cleaner to just expand the value of OUTPUT before we invoke the shell.

Fixes: a8ba798bc8ec ("selftests: enable O and KBUILD_OUTPUT")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/Makefile

@@ -34,34 +34,34 @@ endif
 all: $(SUB_DIRS)
 
 $(SUB_DIRS):
-	BUILD_TARGET=$$OUTPUT/$@; mkdir -p $$BUILD_TARGET; $(MAKE) OUTPUT=$$BUILD_TARGET -k -C $@ all
+	BUILD_TARGET=$(OUTPUT)/$@; mkdir -p $$BUILD_TARGET; $(MAKE) OUTPUT=$$BUILD_TARGET -k -C $@ all
 
 include ../lib.mk
 
 override define RUN_TESTS
 	@for TARGET in $(SUB_DIRS); do \
-		BUILD_TARGET=$$OUTPUT/$$TARGET;	\
+		BUILD_TARGET=$(OUTPUT)/$$TARGET;	\
 		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET run_tests;\
 	done;
 endef
 
 override define INSTALL_RULE
 	@for TARGET in $(SUB_DIRS); do \
-		BUILD_TARGET=$$OUTPUT/$$TARGET;	\
+		BUILD_TARGET=$(OUTPUT)/$$TARGET;	\
 		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET install;\
 	done;
 endef
 
 override define EMIT_TESTS
 	@for TARGET in $(SUB_DIRS); do \
-		BUILD_TARGET=$$OUTPUT/$$TARGET;	\
+		BUILD_TARGET=$(OUTPUT)/$$TARGET;	\
 		$(MAKE) OUTPUT=$$BUILD_TARGET -s -C $$TARGET emit_tests;\
 	done;
 endef
 
 clean:
 	@for TARGET in $(SUB_DIRS); do \
-		BUILD_TARGET=$$OUTPUT/$$TARGET;	\
+		BUILD_TARGET=$(OUTPUT)/$$TARGET;	\
 		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET clean; \
 	done;
 	rm -f tags