imx8mm_beacon: Switch to binman for generating image

To eliminate a warning when using custom imx tools for generating
a binary, use binman to generate flash.bin.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-imx/imx8m/Kconfig b/arch/arm/mach-imx/imx8m/Kconfig
index 276b8bd..a2822f8 100644
--- a/arch/arm/mach-imx/imx8m/Kconfig
+++ b/arch/arm/mach-imx/imx8m/Kconfig
@@ -117,6 +117,7 @@
 
 config TARGET_IMX8MM_BEACON
 	bool "imx8mm Beacon Embedded devkit"
+	select BINMAN
 	select IMX8MM
 	select SUPPORT_SPL
 	select IMX8M_LPDDR4