Instead of counting words in make, which is slightly awful, notice that
possible_devices is local to external_flash.c, so we can declare the array
with an automatic bound, and then get the count as the element-count
(MP_ARRAY_SIZE) of the array.
Since EXTERNAL_FLASH_DEVICE_COUNT is no longer a global macro, switch
a few sites to using EXTERNAL_FLASH_DEVICES in `#if` checks instead.
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