Use a more sensible default library path

When the device has an external SD card, it makes more sense for the
user to store its book there and only use the internal SD card for the
installed applications.

doc/GUIDE.md

@@ -24,7 +24,7 @@ unzip plato-launcher-fmon-VERSION.zip -d SD_ROOT
 
 ## Configure
 
-The default library path is `/mnt/onboard`. If your library lives somewhere else, you'll need to create a file named `Settings.toml` in the same directory as the program's binary with the following content:
+The default library path is `/mnt/onboard` on devices without an external SD card, and `/mnt/sd` otherwise. If your library lives somewhere else, you'll need to create a file named `Settings.toml` in the same directory as the program's binary with the following content:
 ```toml
 library-path = "LIBRARY_PATH"
 ```

src/device.rs

@@ -1,5 +1,6 @@
 use std::env;
 use std::fmt;
+use std::path::PathBuf;
 use std::collections::HashMap;
 use lazy_static::lazy_static;
 use crate::unit::scale_by_dpi;
@@ -69,6 +70,18 @@ pub enum FrontlightKind {
 }
 
 impl Device {
+    pub fn library_path(&self) -> PathBuf {
+        match self.model {
+            Model::AuraH2O |
+            Model::Aura |
+            Model::AuraHD |
+            Model::Mini |
+            Model::Glo |
+            Model::Touch => PathBuf::from("/mnt/sd"),
+            _ => PathBuf::from("/mnt/onboard"),
+        }
+    }
+
     pub fn frontlight_kind(&self) -> FrontlightKind {
         match self.model {
             Model::AuraONE |

src/settings/mod.rs

@@ -251,7 +251,7 @@ impl Default for BatterySettings {
 impl Default for Settings {
     fn default() -> Self {
         Settings {
-            library_path: PathBuf::from("/mnt/onboard"),
+            library_path: CURRENT_DEVICE.library_path(),
             frontlight: true,
             wifi: false,
             rotation_lock: None,