Hello,
You can try to set this boot targets during the boot process.
setenv bootargs 'console=ttymxc2,115200 init=/init video=mxcfb0:dev=ldb,1024x768M@60,bpp=32 video=mxcfb1:off video=mxcfb2:off video=mxcfb3:off vmalloc=400M androidboot.console=ttymxc2 consoleblank=0 androidboot.hardware=freescale cma=384M androidboot.selinux=disabled androidboot.dm_verity=disabled no_console_suspend'
I haven't tested this configuration with an app. But I can write to the serial ports thought shell commands.
I found this web-page that can help you with your problem. I haven't tested the solution, but you can try it and see if it works.
Android* Security Customization with SEAndroid - CodeProject
I hope this can solve your problem.
Regards