I hate to tell you all but there is built in back doors on ALL CPU'S, I find it funny though AMD fans have to post how great there CPU'S R!
If you're worried about backdoors, then there are bigger things to worry about. Windows had have one since the mid 90s, and EFI have a remote patching ability which can be used to add one.
I wouldn't worry about about AMD's BIOS flashing bug, or the various CPU bugs like Spectre, Meltdown and all their siblings. These are all bugs which require you to have direct access or even administrative privileges already (for the BIOS flash). Spectre, Meltdown etc. is basically irrelevant for end users, they require someone to execute
their code on your CPU, find something useful, and then somehow extract that data out of your computer. So it basically needs at least one or two other vulnerabilities to even be theoretically feasible. Don't get me wrong; every bug should be addressed, but that doesn't mean it's a big deal for end users. Bugs like this is also why in IT all good security practices are done by doing security in layers, since bugs in applications/services/libraries, drivers, operating systems, BIOS/EFI or hardware level, or of course
user error are bound to happen sooner or later.
These bugs are primarily a concern for cloud hosting providers, where any random virtual machine may execute the next code on the same CPU. In theory people could extract tiny fragments of memory, and by luck find something useful. But in practice, dumping a hypervisor's 100s of GB of memory at up to a few kB/s, finding and dumping the all-important (and constantly changing) page table of the VM and then assembling continuous application memory page by page, let's just say that the memory you're dumping will change at a much higher rate than your dumping it.
TLDR; You shouldn't care about these bugs. But Intel and AMD should learn from it.