AMD DBM690T and Pistachio work with CBFS

[Carl-Daniel Hailfinger]

A binary search and some log reading revealed the exact issue with CBFS
on AMD DBM690T and AMD Pistachio. The fallback image was called failover
and due to that the CBFS code was unable to find the file
fallback/coreboot_ram and jumped into the void.
Thanks to Stefan for the fix (committed in r4589 and r4590).

The underlying bug is CBFS code jumping to (void *)-1 when a file does
not exist. Log follows:

[...]
CBFS:  Could not find file fallback/coreboot_ram
Jumping to image.
INIT detected from  --- {  APICID = 00 NODEID = 00 COREID = 00} ---
Issuing SOFT_RESET...

Regards,
Carl-Daniel