amd/mandolin: unbreak SeaBIOS VBIOS support

Commit 86ba0d73f3 added VBIOS support for
Raven2 silicon and changed the VBIOS file names to the format including
the PCI device revision number. Upstream SeaBIOS expects the file to
have only the PCI vendor and device IDs in the CBFS file name, so it
doesn't find the VBIOS any more after that patch got applied. This patch
adds the path and CBFS file name to include the Picasso VBIOS a second
time under the CBFS file name SeaBIOS expects.

This is a workaround and not a clean solution, but avoids breakage.
It's separated from the rest of the Mandolin support, so it can just be
reverted after a proper fix is implemented.

https://chromium-review.googlesource.com/2015963/ in combination with a
links file in CBFS might solve the issue for most of the cases, but it's
not sure yet if for all, so a proper fix might require more than that.

BUG=b:153675508

Change-Id: I4d9042615965b6a2d9255c194cf23368264ffe54
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42433
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/amd/mandolin/Kconfig

@@ -91,4 +91,16 @@ config PICASSO_LPC_IOMUX
 	  Picasso's LPC bus signals are MUXed with some of the EMMC signals.
 	  Select this option if LPC signals are required.
 
+#TODO: remove this hack to not break graphics in combination with SeaBIOS
+config VGA_BIOS_DGPU_ID
+	string
+	default "1002,15d8"
+	help
+	  The default VGA BIOS PCI vendor/device ID should be set to the
+	  result of the map_oprom_vendev() function in northbridge.c.
+
+config VGA_BIOS_DGPU_FILE
+	string
+	default "3rdparty/amd_blobs/picasso/PicassoGenericVbios.bin"
+
 endif # BOARD_AMD_MANDOLIN