mainboard: Drop invalid `VGA_BIOS_FILE` defaults

mainboard: Drop invalid `VGA_BIOS_FILE` defaults

If the VGA BIOS file path for `VGA_BIOS_FILE` in a mainboard's Kconfig
does not exist in the coreboot tree (including submodules), drop it.
These files should be stored in the `site-local` subdirectory and the
paths specified for each board in `site-local/Kconfig`. For example:

config VGA_BIOS_FILE
	default "site-local/x200_vbios.bin" if BOARD_LENOVO_X200

Note that this is just an example. There are better ways to structure
one's `site-local` subfolder. Using the `CONFIG_MAINBOARD_DIR` option
would be one of them, though variants may still need special handling.

Also, update autoport to not generate `VGA_BIOS_FILE` defaults.

Change-Id: I1b5dfba035a42d7943f270f95fb7d32b285584d2
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51340
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>