The new option MICROPY_HW_SDCARD_MOUNT_AT_BOOT can now be defined to 0 in
mpconfigboard.h to allow SD hardware to be enabled but not auto-mounted at
boot. This feature is enabled by default to retain previous behaviour.
Previously, if an SD card is enabled in hardware it is also used to boot
from. While this can be disabled with a SKIPSD file on internal flash,
this wont be available at first boot or if the internal flash gets
corrupted.
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