fastboot: Add "--slot other"

This allows you to flash to a slot other than the current one
without needing to enter the name of the slots.

Change-Id: I6bf8c29817be0a29b1abb005f1e72056cee126df
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 5eca845..bd17485 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -318,9 +318,10 @@
             "                                           device supports slots. This will be\n"
             "                                           added to all partition names that use\n"
             "                                           slots. 'all' can be given to refer\n"
-            "                                           to all slots. If this is not given,\n"
-            "                                           slotted partitions will default to\n"
-            "                                           the current active slot.\n"
+            "                                           to all slots. 'other' can be given to\n"
+            "                                           refer to a non-current slot. If this\n"
+            "                                           flag is not used, slotted partitions\n"
+            "                                           will default to the current active slot.\n"
             "  -a, --set-active[=<suffix>]              Sets the active slot. If no suffix is\n"
             "                                           provided, this will default to the value\n"
             "                                           given by --slot. If slots are not\n"
@@ -737,12 +738,28 @@
             if (!suffixes.empty()) {
                 return suffixes[0];
             } else {
-                fprintf(stderr, "No known slots.\n");
-                exit(1);
+                die("No known slots.");
             }
         }
     }
+
     std::vector<std::string> suffixes = get_suffixes(transport);
+
+    if (strcmp(slot, "other") == 0) {
+        std::string current_slot;
+        if (!fb_getvar(transport, "current-slot", &current_slot)) {
+            die("Failed to identify current slot.");
+        }
+        if (!suffixes.empty()) {
+            for (size_t i = 0; i < suffixes.size(); i++) {
+                if (current_slot == suffixes[i])
+                    return suffixes[(i+1)%suffixes.size()];
+            }
+        } else {
+            die("No known slots.");
+        }
+    }
+
     for (const std::string &suffix : suffixes) {
         if (suffix == slot)
             return slot;