Enable A/B system update on marlin/sailfish.

This patch enables the A/B system updater daemon and configures its
basic list of partitions.

Bug: 27347498
Test: Manual procedure, see description.

1. make
2. Flash the produced build with fastboot.
3. make dist
4. Build brillo_update_payload and generate a full payload.
5. Deploying a payload fails because of no HAL, but the daemon starts.

Change-Id: I9928c87f484473802ff0684dcaa22a37d9a3959d

diff --git a/device-common.mk b/device-common.mk
index 3ebd899..f7e644a 100644
--- a/device-common.mk
+++ b/device-common.mk

@@ -248,3 +248,18 @@
 PRODUCT_PACKAGES += \
     keystore.msm8996 \
     gatekeeper.msm8996
+
+# Use the A/B updater.
+AB_OTA_UPDATER := true
+PRODUCT_PACKAGES += \
+   update_engine \
+   update_engine_client \
+   update_verifier
+
+# A/B updater updatable partitions list. Keep in sync with the partition list
+# with "_a" and "_b" variants in the device.
+AB_OTA_PARTITIONS := \
+  boot \
+  system
+
+#TODO(deymo): Add modem and vendor to the previous list.