It turns out that one of our mupdf patch reads the FONTDIR env var
and uses it in a totally different way so we will use another env var
EXT_FONT_DIR to define external font directory for different platforms.
to support custom font directory for EPUB documents
Now Koreader could find fonts in the "fonts" directory in the USB root
directory of kindle, Kobo and PocketBook devices, thus no need to copy
fonts to "koreader/fonts" directory.
