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On i.MX the SysTick IRQ cannot wake the CPU from a WFI so the CPU was
blocked on WFI waiting for USB data in mp_hal_stdin_rx_chr() even though it
had already arrived (because it may arrive just after calling the check
tud_cdc_available()).  This commit fixes this problem by using SEV/WFE to
indicate that there has been a USB event.

The mp_hal_stdout_tx_strn() function is also fixed so that it doesn't
overflow the USB buffers.
Signed-off-by: Damien George <damien@micropython.org>