[make] allow passing a default project to build if none specified and set the build output root

Change-Id: I5241108517be250535329513996c844f68073564

engine.mk

@@ -2,7 +2,7 @@ LOCAL_MAKEFILE:=$(MAKEFILE_LIST)
 
 ifeq ($(MAKECMDGOALS),spotless)
 spotless:
-	rm -rf build-*
+	rm -rf -- build-*
 else
 
 ifndef LKROOT
@@ -32,8 +32,13 @@ endif
 ifeq ($(do-nothing),)
 
 ifeq ($(PROJECT),)
+
+ifneq ($(DEFAULT_PROJECT),)
+PROJECT := $(DEFAULT_PROJECT)
+else
 $(error No project specified.  Use "make projectname" or put "PROJECT := projectname" in local.mk)
 endif
+endif
 
 DEBUG ?= 2