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This allows UART RX to function while flash erase/writes operations are
under way, preventing lost serial data so long as it fits in the UART RX
buffer.

This enables (among other things) mpremote to successfully copy files to
boards that use a UART REPL.

Enable via the following option placed in `mpconfigboard.mk`:

    MICROPY_HW_ENABLE_ISR_UART_FLASH_FUNCS_IN_RAM = 1
Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>