Kobo: Only probe necessary entries in automagic_sysfs codepath

e.g., on the Sage, only the power_dev stuff changes between HW revs,
the rest of the hardcoded stuff is still relevant, so skip probing
those.

frontend/device/kobo/device.lua

@@ -627,15 +627,18 @@ function Kobo:init()
     -- Automagic sysfs discovery
     if self.automagic_sysfs then
         -- Battery
+        if not self.battery_sysfs then
             if util.pathExists("/sys/class/power_supply/battery") then
                 -- Newer devices (circa sunxi)
                 self.battery_sysfs = "/sys/class/power_supply/battery"
             else
                 self.battery_sysfs = "/sys/class/power_supply/mc13892_bat"
             end
+        end
 
         -- Frontlight
         if self:hasNaturalLight() then
+            if not self.frontlight_settings.frontlight_mixer then
                 if util.fileExists("/sys/class/leds/aw99703-bl_FL1/color") then
                     -- HWConfig FL_PWM is AW99703x2
                     self.frontlight_settings.frontlight_mixer = "/sys/class/leds/aw99703-bl_FL1/color"
@@ -647,8 +650,10 @@ function Kobo:init()
                     self.frontlight_settings.frontlight_mixer = "/sys/class/backlight/tlc5947_bl/color"
                 end
             end
+        end
 
-        -- Input
+        -- Touch panel input
+        if not self.touch_dev then
             if util.fileExists("/dev/input/by-path/platform-1-0010-event") then
                 -- Elan (HWConfig TouchCtrl is ekth6) on i2c bus 1
                 self.touch_dev = "/dev/input/by-path/platform-1-0010-event"
@@ -658,7 +663,10 @@ function Kobo:init()
             else
                 self.touch_dev = "/dev/input/event1"
             end
+        end
 
+        -- Physical buttons & synthetic NTX events
+        if not self.ntx_dev then
             if util.fileExists("/dev/input/by-path/platform-gpio-keys-event") then
                 -- Libra 2 w/ a BD71828 PMIC
                 self.ntx_dev = "/dev/input/by-path/platform-gpio-keys-event"
@@ -671,15 +679,19 @@ function Kobo:init()
             else
                 self.ntx_dev = "/dev/input/event0"
             end
+        end
 
+        -- Power button (this usually ends up in ntx_dev, except with some PMICs)
+        if not self.power_dev then
             if util.fileExists("/dev/input/by-path/platform-bd71828-pwrkey-event") then
                 -- Libra 2 w/ a BD71828 PMIC
                 self.power_dev = "/dev/input/by-path/platform-bd71828-pwrkey-event"
             elseif util.fileExists("/dev/input/by-path/platform-bd71828-pwrkey.4.auto-event") then
-            -- Fix for Kobo Sage, presumably w/ a BD71828 PMIC?
+                -- Sage w/ a BD71828 PMIC
                 self.power_dev = "/dev/input/by-path/platform-bd71828-pwrkey.4.auto-event"
             end
         end
+    end
 
     -- NOTE: i.MX5 devices have a wonky RTC that doesn't like alarms set further away that UINT16_MAX seconds from now...
     --       (c.f., WakeupMgr for more details).