imx: hab: Use nxp_imx8mcst etype for i.MX8M flash.bin signing

Update documentation and use nxp_imx8mcst binman etype for signing
of flash.bin instead of previous horrible shell scripting.

Reviewed-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Marek Vasut <marex@denx.de>
4 files changed