fastbootd: Implement getvar hw-revision.

Bug: 78793464
Test: fastboot getvar hw-revision works
      fuzzy_fastboot Conformance.GetVarRevision passes

Change-Id: I5a3e9893d61e18567f2f818ef06cad8e862d637f
diff --git a/fastboot/device/commands.cpp b/fastboot/device/commands.cpp
index b1c2958..771c288 100644
--- a/fastboot/device/commands.cpp
+++ b/fastboot/device/commands.cpp
@@ -89,7 +89,8 @@
             {FB_VAR_SLOT_UNBOOTABLE, {GetSlotUnbootable, nullptr}},
             {FB_VAR_PARTITION_SIZE, {GetPartitionSize, GetAllPartitionArgsWithSlot}},
             {FB_VAR_IS_LOGICAL, {GetPartitionIsLogical, GetAllPartitionArgsWithSlot}},
-            {FB_VAR_IS_USERSPACE, {GetIsUserspace, nullptr}}};
+            {FB_VAR_IS_USERSPACE, {GetIsUserspace, nullptr}},
+            {FB_VAR_HW_REVISION, {GetHardwareRevision, nullptr}}};
 
     if (args.size() < 2) {
         return device->WriteFail("Missing argument");