CONFIG_BUILD_KERNEL/libapps.a: Allow creation of libapps.a separately

Add rule to create libapps.a for CONFIG_BUILD_KERNEL=y, there is no
reason to disallow this.

The behavior of make import remains the same, so no regression is expected.

Makefile

@@ -62,10 +62,12 @@ ifeq ($(CONFIG_BUILD_KERNEL),y)
 
 install: $(foreach SDIR, $(CONFIGURED_APPS), $(SDIR)_install)
 
-.import: $(foreach SDIR, $(CONFIGURED_APPS), $(SDIR)_all)
+$(BIN): $(foreach SDIR, $(CONFIGURED_APPS), $(SDIR)_all)
 	$(Q) for app in ${CONFIGURED_APPS}; do \
 		$(MAKE) -C "$${app}" archive ; \
 	done
+
+.import: $(BIN)
 	$(Q) install libapps.a $(APPDIR)$(DELIM)import$(DELIM)libs
 	$(Q) $(MAKE) install