The SystemReady ACS tests are generally run on a bare metal system. Can the ACS tests be run under a U-Boot bootloader shell

Yes, you can run the compliance test on a U-Boot based system, but there are some considerations to be aware of:

• BSA ACS does support compilation for U-Boot based environments. This means you can run compliance tests for your U-Boot system using BSA ACS. The U-Boot environment means that the system firmware is exposing Device Tree and not the ACPI table.
• SBSA ACS does not support U-Boot based systems. SBSA ACS is specifically designed for UEFI-based systems, which are more typical in server-based environments. If your end goal is to obtain SystemReady certification for a server-based system, it is recommended to use SBSA ACS. System Ready certification is essential for server deployments, and SBSA ACS is the appropriate tool to ensure compliance with the required standards.

To run BSA ACS with your U-Boot system, you can follow the instructions provided in the BSA ACS repository for IoT Ready (IR) systems: https://github.com/ARM-software/bsa-acs/tree/main#for-ir-systems .  For convenience, you can find pre-built images of the latest BSA ACS version at the following location: https://github.com/ARM-software/bsa-acs/tree/main/prebuilt_images/IR/v23.09_REL1.0.6 

Note The ACS test suite runs on top of a UEFI Shell.  Therefore to run on a U-Boot system, a UEFI shell needs to be loaded manually from the U-Boot terminal, so that the BSA binary can run.  The UEFI shell can be built separately, or a prebuilt image is currently available at https://github.com/ARM-software/bsa-acs/tree/main/prebuilt_images/IR/v23.09_REL1.0.6.