Add clear_flags command

As the name suggests, it clears all the flags in the configuration
buffer.  This provides a way to easily set all the flags to a known
state, which is nice for implementing libraries that configure PD Buddy
Sink devices.

docs/console_config.md

 Usage: `load`
 
 Loads the current configuration from flash into the buffer.  Useful if you want
-to change some settings while leaving others alone.
+to change some settings while leaving others alone.  If there is no
+configuration, `No configuration` is printed instead.
 
 ### get_cfg
 
 
 Prints the contents of the configuration buffer.
 
+### clear_flags
+
+Usage: `clear_flags`
+
+Clears all the flags in the configuration buffer.
+
 ### toggle_giveback
 
 Usage: `toggle_giveback`

src/shell.c

     pdb_config_print(chp, &tmpcfg);
 }
 
+static void cmd_clear_flags(BaseSequentialStream *chp, int argc, char *argv[])
+{
+    (void) argv;
+    if (argc > 0) {
+        chprintf(chp, "Usage: clear_flags\r\n");
+        return;
+    }
+
+    /* Clear all flags */
+    tmpcfg.flags = 0;
+}
+
 static void cmd_toggle_giveback(BaseSequentialStream *chp, int argc, char *argv[])
 {
     (void) argv;
     {"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"},
+    {"clear_flags", cmd_clear_flags, "Clear all flags"},
     {"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"},