fastbootd: Add getvar is-force-debuggable

Export ro.force.debuggable property so that the user can check image
compatibility prior to running fastboot flash.
For example, if is-force-debuggable is yes, then the "system" and
"vendor" build fingerprint must match.

Bug: 191649082
Test: fastboot getvar is-force-debuggable
Change-Id: I772d98253f58ba208d5803e18b589ff693deebd0
diff --git a/fastboot/device/variables.cpp b/fastboot/device/variables.cpp
index 5f99656..d2a7947 100644
--- a/fastboot/device/variables.cpp
+++ b/fastboot/device/variables.cpp
@@ -379,6 +379,12 @@
     return true;
 }
 
+bool GetIsForceDebuggable(FastbootDevice* /* device */, const std::vector<std::string>& /* args */,
+                          std::string* message) {
+    *message = android::base::GetBoolProperty("ro.force.debuggable", false) ? "yes" : "no";
+    return true;
+}
+
 std::vector<std::vector<std::string>> GetAllPartitionArgsWithSlot(FastbootDevice* device) {
     std::vector<std::vector<std::string>> args;
     auto partitions = ListPartitions(device);