build: add dummy @: to some "all" targets

This suppresses:

make[3]: Nothing to be done for 'all'.

when nothing needs to be rebuilt.

Still more work will be needed, specifically in
examples and apps directories.  But this will cover
most of lib/ and anything in unit/test that is using
the new spdk.unittest.mk.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: I1c0c2b88e70ec37629740aa47f612c9d46fc88fb

Reviewed-on: https://review.gerrithub.io/399725
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>

mk/spdk.lib.mk

@@ -38,6 +38,7 @@ LIB := $(call spdk_lib_list_to_files,$(LIBNAME))
 .PHONY: all clean $(DIRS-y)
 
 all: $(LIB) $(DIRS-y)
+	@:
 
 clean: $(DIRS-y)
 	$(CLEAN_C) $(LIB)

mk/spdk.unittest.mk

@@ -47,6 +47,7 @@ LIBS += -lcunit $(SPDK_LIB_LINKER_ARGS)
 APP = $(TEST_FILE:.c=)
 
 all: $(APP)
+	@:
 
 $(APP) : $(OBJS) $(SPDK_LIB_FILES)
 	$(LINK_C)