bootsplash: Add ImageMagick voodoo

The JPEG decoder, that was added many years ago to display a boot-
splash in coreboot, has a few quirks. People used to do some voodoo
with GIMP to convert images to the right format, but we can also
achieve the same with ImageMagick's `convert`. The currently known
constraints are:
* The framebuffer's color format is ignored,
* only YCC 4:2:0 color sampling is supported, and
* width and height have to be a multiple of 16 pixels.

Beside that, we can only display the bootsplash if it completely
fits into the framebuffer. As the latter's size is often decided
at runtime, we can't do much more than offering an option to set
a specific size.

Change-Id: I564e0d89fb46503ff4c11e095726616700009968
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76564
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>

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