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/constants.h b/fastboot/constants.h
index b780ba5..43daab0 100644
--- a/fastboot/constants.h
+++ b/fastboot/constants.h
@@ -57,3 +57,4 @@
 #define FB_VAR_SLOT_SUCCESSFUL "slot-successful"
 #define FB_VAR_SLOT_UNBOOTABLE "slot-unbootable"
 #define FB_VAR_IS_LOGICAL "is-logical"
+#define FB_VAR_IS_USERSPACE "is-userspace"