feat(imx8mq): make IMX_BOOT_UART_BASE configurable via build parameter

This aligns the i.MX8MQ build with the other i.MX8M platforms by
allowing to override the default IMX_BOOT_UART_BASE value via a make
parameter.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Change-Id: Iad9b844517209fc7d051c61767f71ac9fa2b55c7
2 files changed