fastbootd: Add fetch command on device

Test: manual
Test: see follow up CL on fuzzy_fastboot
Bug: 173654501

Change-Id: I912d60d6dca0082734f2b84adc6a647c881bb5a1
diff --git a/fastboot/constants.h b/fastboot/constants.h
index 4cc154a..4ea68da 100644
--- a/fastboot/constants.h
+++ b/fastboot/constants.h
@@ -35,6 +35,7 @@
 #define FB_CMD_OEM "oem"
 #define FB_CMD_GSI "gsi"
 #define FB_CMD_SNAPSHOT_UPDATE "snapshot-update"
+#define FB_CMD_FETCH "fetch"
 
 #define RESPONSE_OKAY "OKAY"
 #define RESPONSE_FAIL "FAIL"