video/logo: allow custom logo

Some people like to replace the default Tux boot logo by an image of
their own. There exist a few tutorials here [1] and there [2]. But
this requires modifying the source tree which is a bit cumbersome.

Add a string entry in Kbuild for each of the logo categories
(monochrome, 16-colors, 224-colors). The string entry takes a path to
a .pbm or .ppm image allowing the user to more easily provide a custom
logo without having to modify the sources.

Add an help entry with a short hint on how to convert images to the
portable pixmap file format.

Update the Makefile accordingly. When converted to .c file, the logo
will have one of these fixed file name:

  - logo_linux_mono.c
  - logo_linux_vga16.c
  - logo_linux_clut224.c:

depending on the image type and this regardless of the name of the
.pgm/.ppm source filename. This will allow for further simplifications
in an upcoming change.

[1] ArmadeuS Project wiki -- Linux Boot Logo
Link: https://www.armadeus.org/wiki/index.php?title=Linux_Boot_Logo

[2] Timesys -- How To Use a Custom Boot Logo / Splash Screen
Link: https://linuxlink.timesys.com/docs/wiki/engineering/HOWTO_Use_a_custom_boot_logo

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>

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