Added toggle_giveback shell command

As the name says, it toggles the GiveBack flag in the configuration
buffer.  The rest of the firmware doesn't use the flag for anything yet,
but it is correctly saved in flash.

src/shell.c

     pdb_config_print(chp, &tmpcfg);
 }
 
+static void cmd_toggle_giveback(BaseSequentialStream *chp, int argc, char *argv[])
+{
+    (void) argv;
+    if (argc > 0) {
+        chprintf(chp, "Usage: toggle_giveback\r\n");
+        return;
+    }
+
+    /* Toggle the GiveBack flag */
+    tmpcfg.flags ^= PDB_CONFIG_FLAGS_GIVEBACK;
+}
+
 static void cmd_set_v(BaseSequentialStream *chp, int argc, char *argv[])
 {
     if (argc != 1) {
     {"load", cmd_load, "Load the stored configuration into the buffer"},
     {"get_cfg", cmd_get_cfg, "Print the stored configuration"},
     {"get_tmpcfg", cmd_get_tmpcfg, "Print the configuration buffer"},
-    /* TODO {"toggle_giveback", cmd_toggle_giveback, "Toggle the GiveBack flag"},*/
+    {"toggle_giveback", cmd_toggle_giveback, "Toggle the GiveBack flag"},
     /* TODO {"toggle_var_bat", cmd_toggle_var_bat, "Toggle the Var/Bat flag"},*/
     {"set_v", cmd_set_v, "Set the voltage in millivolts"},
     {"set_i", cmd_set_i, "Set the current in milliamps"},