console/font: Refactor font support code selection logic

console/font: Refactor font support code selection logic

The current Makefile rules to build font support are messy and buggy.
Replace them by Kconfig rules:
  - Introduce CONFIG_FONT_SUPPORT, which controls the building of all font
    code,
  - Select CONFIG_FONT_SUPPORT for all drivers that use fonts,
  - Select CONFIG_FONT_8x16 for all drivers that default to the VGA8x16
    font,
  - Drop the bogus console dependency for CONFIG_VIDEO_VIVI,
  - Always process drivers/video/console/Makefile, as some drivers need
    fonts even if CONFIG_VT is not set.

This fixes (if CONFIG_SOLO6X10=y and there are no built-in console
drivers):

drivers/built-in.o: In function `solo_osd_print':
drivers/staging/media/solo6x10/solo6x10-enc.c:144: undefined reference to `.find_font'

This fixes (if CONFIG_VT=n):

drivers/built-in.o: In function `vivi_init':
vivi.c:(.init.text+0x1a3da): undefined reference to `find_font'

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com> [original part]
Acked-by: Randy Dunlap <rdunlap@infradead.org> [drivers/video/Makefile]