[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
 

makefile

@@ -5,13 +5,22 @@
 LKMAKEROOT ?= .
 LKROOT ?= .
 LKINC ?=
+BUILDROOT ?= .
+DEFAULT_PROJECT ?=
 TOOLCHAIN_PREFIX ?=
 
 LKINC := $(LKROOT) $(LKINC)
 
+export LKMAKEROOT
+export LKROOT
+export LKINC
+export BUILDROOT
+export DEFAULT_PROJECT
+export TOOLCHAIN_PREFIX
+
 # vaneer makefile that calls into the engine with lk as the build root
 # if we're the top level invocation, call ourselves with additional args
 $(MAKECMDGOALS) _top:
-	TOOLCHAIN_PREFIX=$(TOOLCHAIN_PREFIX) LKROOT=$(LKROOT) LKINC="$(LKINC)" $(MAKE) -C $(LKMAKEROOT) -rR -f $(LKROOT)/engine.mk $(addprefix -I,$(LKINC)) $(MAKECMDGOALS)
+	@$(MAKE) -C $(LKMAKEROOT) -rR -f $(LKROOT)/engine.mk $(addprefix -I,$(LKINC)) $(MAKECMDGOALS)
 
 .PHONY: _top