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/device/variables.h b/fastboot/device/variables.h
index 63f2670..0546942 100644
--- a/fastboot/device/variables.h
+++ b/fastboot/device/variables.h
@@ -52,6 +52,7 @@
                     std::string* message);
 bool GetHardwareRevision(FastbootDevice* device, const std::vector<std::string>& args,
                          std::string* message);
+bool GetVariant(FastbootDevice* device, const std::vector<std::string>& args, std::string* message);
 
 // Helpers for getvar all.
 std::vector<std::vector<std::string>> GetAllPartitionArgsWithSlot(FastbootDevice* device);