lib/fonts: Remove internal symbols and macros from public header file

Define access macros for font_data_t in fonts.c. Define struct font_data
and declare most of the font symbols in the internal header font.h, where
they can only be seen by the font code.

Also move font indices into internal font.h. They appear to be unused
though. There is m86k assembly code that operates on the idx field, so
leave them in place for now.

List all built-in fonts in a separate section in the public header file.

v2:
- do not add config guards around font symbols (Helge)
- keep declaration of built-in fonts in public header

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>