Add arch.mk

- Add arch.mk, which is included from build/preamble.mk
- Define cache paths
- Add empty BOARD_PACKAGES_EXTRA
- Define BOOT/ROOTFS sizes

Change-Id: Ib04e3b74ee6d489875018d32b2c8a9b620dae82a
diff --git a/arch.mk b/arch.mk
new file mode 100644
index 0000000..8bc51e9
--- /dev/null
+++ b/arch.mk
@@ -0,0 +1,22 @@
+# Copyright 2019 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+ifeq ($(IS_EXTERNAL),)
+  ROOTFS_RAW_CACHE_DIRECTORY ?= /google/data/ro/teams/spacepark/enterprise/kokoro/prod/spacepark/enterprise/rootfs/latest/
+endif
+
+BOARD_PACKAGES_EXTRA :=
+
+BOOT_SIZE_MB := 128
+ROOTFS_SIZE_MB := 4096