[coreboot] [PATCH]Use cbfs-files-y for normal file additions

[Georgi, Patrick]

Hi,

we currently have special cases for VGABIOS, Intel MBI and
splashscreens. Use cbfs-files-y instead, which provides a canonical way
for adding raw files to CBFS.

Question is, if we should drop these Kconfig options entirely and let
the user handle these entries in the board's Makefile.inc?

Signed-off-by: Patrick Georgi