flashy-95a150a

detect missing/broken flash chips on non-vboot

Summary:
If the U-Boot environment (env MTD partition) is inaccessible and flash0 is
missing too, it's a good indication that something serious is amiss with the
SPI chips.  Return a  broken flash chip error code in this case, instead of
an unsafe to reboot error.

(For example an A-SPEED/Linux driver bug may hide the env partition on the
primary chip invisible if the secondary chip is inaccessible.)

Test Plan:
Build & unit test:

```
0 ~/local/openbmc/tools/flashy $ ./build.sh && ./build_dev.sh && go test ./...
ok      github.com/facebook/openbmc/tools/flashy        (cached)
ok      github.com/facebook/openbmc/tools/flashy/checks_and_remediations/common (cached)
ok      github.com/facebook/openbmc/tools/flashy/checks_and_remediations/galaxy100      (cached)
ok      github.com/facebook/openbmc/tools/flashy/checks_and_remediations/wedge100       (cached)
ok      github.com/facebook/openbmc/tools/flashy/checks_and_remediations/yamp   (cached)
?       github.com/facebook/openbmc/tools/flashy/flash_procedure        [no test files]
ok      github.com/facebook/openbmc/tools/flashy/install        (cached)
ok      github.com/facebook/openbmc/tools/flashy/lib/fileutils  (cached)
ok      github.com/facebook/openbmc/tools/flashy/lib/flash      (cached)
ok      github.com/facebook/openbmc/tools/flashy/lib/flash/flashcp      (cached)
ok      github.com/facebook/openbmc/tools/flashy/lib/flash/flashutils   (cached)
ok      github.com/facebook/openbmc/tools/flashy/lib/flash/flashutils/devices   (cached)
?       github.com/facebook/openbmc/tools/flashy/lib/logger     [no test files]
ok      github.com/facebook/openbmc/tools/flashy/lib/step       (cached)
ok      github.com/facebook/openbmc/tools/flashy/lib/utils      (cached)
ok      github.com/facebook/openbmc/tools/flashy/lib/validate   (cached)
ok      github.com/facebook/openbmc/tools/flashy/lib/validate/image     (cached)
ok      github.com/facebook/openbmc/tools/flashy/lib/validate/partition (cached)
?       github.com/facebook/openbmc/tools/flashy/tests  [no test files]
?       github.com/facebook/openbmc/tools/flashy/utilities      [no test files]
0 ~/local/openbmc/tools/flashy $ echo $?
0
```

Reviewed By: kawmarco

fbshipit-source-id: 355a7d01ce55f4994ca76693b360592206651b6a