fastbootd: exporting CPU ABI info

CPU ABI info, e.g., arm64-v8a, can be useful to determine image
compatibility, prior to flash. Adding this info in fastbootd.

Bug: 74445765
Test: fastboot getvar cpu-abi # arm64-v8a
Change-Id: Ied494b646c551320295956b7890c0102fdb88382
diff --git a/fastboot/constants.h b/fastboot/constants.h
index 7fba67c..5a554a0 100644
--- a/fastboot/constants.h
+++ b/fastboot/constants.h
@@ -68,3 +68,4 @@
 #define FB_VAR_BATTERY_SOC_OK "battery-soc-ok"
 #define FB_VAR_SUPER_PARTITION_NAME "super-partition-name"
 #define FB_VAR_SNAPSHOT_UPDATE_STATUS "snapshot-update-status"
+#define FB_VAR_CPU_ABI "cpu-abi"