fastbootd: Add is-userspace variable.

Bug: 78793464
Test: fastboot getvar is-userspace
          returns true for fastbootd
      fastboot getvar is-userspace
          returns false or error for bootloader fastboot.

Change-Id: I80e1ef6dcf3922e4f555cb2e409e3e098c16fbbc
diff --git a/fastboot/device/commands.cpp b/fastboot/device/commands.cpp
index 053d654..690bfa8 100644
--- a/fastboot/device/commands.cpp
+++ b/fastboot/device/commands.cpp
@@ -59,7 +59,8 @@
             {FB_VAR_SLOT_SUCCESSFUL, GetSlotSuccessful},
             {FB_VAR_SLOT_UNBOOTABLE, GetSlotUnbootable},
             {FB_VAR_PARTITION_SIZE, GetPartitionSize},
-            {FB_VAR_IS_LOGICAL, GetPartitionIsLogical}};
+            {FB_VAR_IS_LOGICAL, GetPartitionIsLogical},
+            {FB_VAR_IS_USERSPACE, GetIsUserspace}};
 
     // args[0] is command name, args[1] is variable.
     auto found_variable = kVariableMap.find(args[1]);