charger: Ensure every generated image is 8-bit

minui supports only <= 8-bit PNGs.

On my machine battery_fail was being encoded
using 24-bits and charger was freaking out :(

Change-Id: I2f63548ba102f49485c43268389f1cdd5e50802d
Signed-off-by: Luca Stefani <luca.stefani.ge1@gmail.com>
(cherry picked from commit 3c1a396159d02f586b92ce3c37f6e78db1ae1c43)

charger/regenerate.sh

@@ -14,7 +14,7 @@ for DENSITY in mdpi:160 hdpi:240 xhdpi:320 xxhdpi:480 xxxhdpi:640; do
 
     for SVG in svg/*.svg; do
         PNG="$DPI/$(basename $SVG | cut -f1 -d '.').png"
-        convert -density $WIDTH -resize ${WIDTH}x${WIDTH} $SVG $PNG
+        convert -density $WIDTH -resize ${WIDTH}x${WIDTH} $SVG png24:$PNG
     done
 
     SCALEFILE="$DPI/battery_scale.png"