build: Generate a staging folder like kernel side

so the user could generate the separate library through BIN.
Here is the similar code snippet copied from nuttx/tools/Unix.mk:
context: include/nuttx/config.h include/nuttx/version.h .dirlinks $(CONTEXTDIRS_DEPS) | staging

staging:
	$(Q) mkdir -p $@

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

.gitignore

@@ -26,6 +26,7 @@
 /external
 /Kconfig
 /romfs.img
+/staging
 /symtab_apps.c
 cscope.out
 Make.dep

Makefile

@@ -158,7 +158,10 @@ dirlinks:
 context_all: $(foreach SDIR, $(CONFIGURED_APPS), $(SDIR)_context)
 register_all: $(foreach SDIR, $(CONFIGURED_APPS), $(SDIR)_register)
 
-context:
+staging:
+	$(Q) mkdir -p $@
+
+context: | staging
 	$(Q) $(MAKE) context_all
 	$(Q) $(MAKE) register_all