shared/filesystem: mount SD at filesystem_init for reliable MSC probe

The lazy automount in tud_msc_test_unit_ready_cb can lose races with
macOS's USB MSC probe timing -- the host asks whether LUN 1 is ready
before the card has finished mounting, sees NOT_READY, and may give up
before trying again. Mounting during filesystem_init ensures the SD
card is live by the time USB enumerates, so the first probe succeeds.

Affects boards that define DEFAULT_SD_CARD_DETECT; behavior on boards
without an auto-mount path is unchanged.