apalis-imx8: enable i.MX specific BOOTAUX

commit e8cd1f60 ("imx: imx8: bootaux: Add i.MX8 M4 boot support")
implemented IMX_BOOTAUX. Enable it.

Signed-off-by: Andrejs Cainikovs <andrejs.cainikovs@toradex.com>
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>