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/constants.h b/fastboot/constants.h
index 063cd40..8a425ae 100644
--- a/fastboot/constants.h
+++ b/fastboot/constants.h
@@ -57,3 +57,4 @@
 #define FB_VAR_SLOT_UNBOOTABLE "slot-unbootable"
 #define FB_VAR_IS_LOGICAL "is-logical"
 #define FB_VAR_IS_USERSPACE "is-userspace"
+#define FB_VAR_HW_REVISION "hw-revision"