docs: Only 5570 and 8587 are valid EC IDs

ecflash has no verification for the EC it detects. It will attempt to
read and erase whatever it finds, where it will hang. Document what it
*should* read and an example of what it *shouldn't* read.

Signed-off-by: Tim Crawford <[email protected]>

Author: crawfxrd

doc/flashing.md

@@ -43,3 +43,18 @@ This requires:
 ```
 make BOARD=<vendor>/<model> flash_external
 ```
+
+One of the first things it does is read the EC ID and version. If working, the
+ID will be the EC model the machine uses.
+
+```
+ID: 5570 VER: 2
+ID: 8587 VER: 6
+```
+
+Any other values means that the Mega 2560 is misconfigured or the FPC is not
+seated correctly. E.g., this is wrong:
+
+```
+ID: FF7F VER: 127
+```