Push some definitions out of preamble.mk into board/

- Require including board/arch.mk
- Move overrides of prebuilts into board/arch.mk to allow per-board
choices
- Move BOOT/ROOTFS_SIZE definitions into board/arch.mk
- Append BOARD_PACKAGES_EXTRA to PACKAGES_EXTRA, to allow per-board
preinstalled packages

Change-Id: I76f00e03036d56bcfd3246c271413b51d2871332
diff --git a/preamble.mk b/preamble.mk
index b223373..26e188b 100644
--- a/preamble.mk
+++ b/preamble.mk
@@ -22,14 +22,13 @@
   IS_EXTERNAL ?= true
 endif
 endif
-
-FETCH_PACKAGES ?= false
+-include $(ROOTDIR)/board/arch.mk
 ifeq ($(IS_EXTERNAL),)
   PREBUILT_DOCKER_ROOT ?= /google/data/ro/teams/spacepark/enterprise/kokoro/prod/spacepark/enterprise/docker/
   FETCH_PBUILDER_DIRECTORY ?= /google/data/ro/teams/spacepark/enterprise/kokoro/prod/spacepark/enterprise/pbuilder/
-  ROOTFS_RAW_CACHE_DIRECTORY ?= /google/data/ro/teams/spacepark/enterprise/kokoro/prod/spacepark/enterprise/rootfs/latest/
 endif
 
+FETCH_PACKAGES ?= false
 USERSPACE_ARCH ?= arm64
 
 # Architecture specific defines here
@@ -142,5 +141,4 @@
 	xdg-user-dirs \
 	xwayland
 
-BOOT_SIZE_MB := 128
-ROOTFS_SIZE_MB := 4096
+PACKAGES_EXTRA += $(BOARD_PACKAGES_EXTRA)