build/resource.mk: Fix bmp -> png conversion

ImageMagick 6.8.6 ignored the "png:color-type" ("convert: Cannot write image with
defined png:bit-depth or png:color-type.") and wrote color pngs instead
leading to the assertion:

   xcsoar: src/Screen/OpenGL/UncompressedImage.cpp:70:
     GLTexture* ImportAlphaTexture(const UncompressedImage&):
     Assertion `image.GetFormat() == UncompressedImage::Format::GRAY' failed.

adding "-type GrayScale" fixes the issue.
Manual: Reduce button styles from "de" branch to a more generic set.
Manual en flash: Tiny fix on XC not providing collision info.