imx: use BOOT_DEVICE_BOARD instead of UART

i.MX 6 serial downloader is not necessarily booting via UART but can
also boot from USB. In fact only some i.MX chips have serial
downloader support via UART (e.g. 6UL/ULL and Vybrid) but all of
them have serial downloader support via USB. Use the more appropriate
BOOT_DEVICE_BOARD define which is used for ROM provided recovery
mechanisms in general.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>