Support fastboot variable 'variant'.

Bug: 79480454
Bug: 78793464
Test: fastboot getvar variant
Change-Id: Iefef82c147d8405318c793ad0a73f00674bbb63d
Merged-In: Iefef82c147d8405318c793ad0a73f00674bbb63d
(cherry picked from commit 4785287a0043ddea2f784433d8a4fc8a168efc79)
diff --git a/fastboot/constants.h b/fastboot/constants.h
index 2a68a2b..ad2e1a1 100644
--- a/fastboot/constants.h
+++ b/fastboot/constants.h
@@ -60,3 +60,4 @@
 #define FB_VAR_IS_LOGICAL "is-logical"
 #define FB_VAR_IS_USERSPACE "is-userspace"
 #define FB_VAR_HW_REVISION "hw-revision"
+#define FB_VAR_VARIANT "variant"