rgnano: Instant play support and proper system brightness/volume handling

Instant play is a Funkey OS feature that allows it to relaunch at boot the last program it was running before shutting down, which means that if the handheld is powered off (holding the power button) while rockbox is running the next time it's powered on rockbox will launch at boot.

This commit also handles system brightness/volume in rockbox itself instead of the launch script, so the values are properly reset when powering off the handheld while rockbox is running.

Change-Id: Ie1adbf71069aeed5fbf6670971718a2f718716a3

packaging/rgnano/run.sh

@@ -4,6 +4,11 @@ RBDIR=/mnt/FunKey/rockbox
 CFGFILE=$RBDIR/config.cfg
 BLPATH=/sys/class/backlight/backlight/brightness
 
+_send_sigusr1()
+{
+  kill -s USR1 "$rb_pid" 2>/dev/null
+}
+
 # Install the rockbox folder
 if [ ! -d $RBDIR ]; then
   notif set 0 " Installing rockbox..."
@@ -27,12 +32,13 @@ if [ ! -f $CFGFILE ]; then
   cp ./config.cfg $CFGFILE
 fi
 
-# Get current volume/brightness -> launch rockbox -> restore previous values
-CUR_VOL=$(volume get)
-CUR_BL=$(cat $BLPATH)
-volume set 100
+# Set volume to max with amixer so it's not permanent
+amixer -q sset 'Headphone' 63 unmute
 
-./rockbox
+# Need to send SIGUSR1 to the rockbox process for instant play support
+trap _send_sigusr1 SIGUSR1
 
-volume set $CUR_VOL
-echo $CUR_BL > $BLPATH
+./rockbox &
+
+rb_pid=$!
+wait "$rb_pid"