Unable to enable kgdb on DiGi EL

My board is running DiGi EL on ccardwmx28js.
Have compiled the kernel with the following options enabled (as built-in modules)

Problem 1) Runtime configuration of kgdboc is failing
After flashing the Linux image I can see kgdboc module in /sys directory. But unable to enable it.

Following error is thrown when tried to enable it
/ # echo ttyAM0,115200 > sys/module/kgdboc/parameters/kgdboc
sh: write error: No such device

But ttyAM0 is functioning well as debug port (DUART).
Not sure how to solve this problem. Could anyone please guide me?

Problem 2) How to enable kgdboc as kernel boot argument?
DiGi EL provided uboot by default use the “dboot linux flash” as the bootcmd.
I am trying to figure out how to append kernel boot argument kgdboc=ttyAM0,115200 to this.
If I try to use environment variable std_bootarg it is appending kgdboc=ttyAM0,115200 to the end of the string.

/ # cat /proc/cmdline
gpmi console=ttyAM0,115200 root=/dev/mtdblock3 rootfstype=jffs2 rw mtdparts=gpmi
S) video=displayfb:LCD@LQ070Y3DG3B fbprimary=video kgdbwait kgdboc=ttyAM0,11520

This is not enabling the kgdboc. Can be confirmed by following command
/ # cat /sys/module/kgdboc/parameters/kgdboc
/ #

Neither can enable it
/ # echo ttyAM0,115200 > /sys/module/kgdboc/parameters/kgdboc
sh: write error: No such device

Could anyone please tell me how to enable kgdboc on DiGi EL?