Hello Sreedhar,
In i.MX_Linux_User's_Guide.pdf, it is described on how to operate standalone mode and how to export tool chain.
you can do like this:
--->compiling image
1. DISTRO=fsl-imx-xwayland MACHINE=imx8mmevk source fsl-setup-release.sh -b build-xwayland
2. bitbake core-image-minimal
--->exporting tool chain
1.Using the command to export toolchain SDK.
#DISTRO=fsl-imx-xwayland MACHINE=imx8mmevk bitbake core-image-minimal -c populate_sdk
Then you will get a .sh file including tool chain, see below, please!

2. Copy the file to /opt/
# cd ~/imx-yocto-bsp/build-xwayland/tmp/deploy/sdk
# sudo cp ./fsl-imx-xwayland-glibc-x86_64-core-image-minimal-aarch64-toolchain-4.14-sumo.sh /opt
# cd /opt
# sudo ././fsl-imx-xwayland-glibc-x86_64-core-image-minimal-aarch64-toolchain-4.14-sumo.sh
Then according to guidance to install toolchain to /opt directory or other director you like.
Below is mine:

3. Before starting to opreate standalone, environment must be set.
Below is linux and u-boot source code on my ubuntu:

Set environment:
# source /opt/Cortex-A53-L4.14.98-2.0.0-toolchain/4.14-sumo/environment-setup-aarch64-poky-linux
The command indicates the CROSS_COMPILE environment has been ready.
Now you can use "make" to compile u-boot or linux source code.
Hope above contents are helpful to you!
Have a nice day!
B.R,
Weidong