fix UHDAtomics.cmake to read custom Boost install prefix

[icyveins7]

Pull Request Details

Description

This fixes a CMake build issue with custom prefix-installed Boost libraries.

Related Issue

Resolves #632

Which devices/areas does this affect?

NA, this is a build issue.

Testing Done

CMake proceeds.

Checklist

• [ x] I have read the CONTRIBUTING document.
• [ x] My code follows the code style of this project. See CODING.md.
• [ x] I have updated the documentation accordingly.
• [ x] I have added tests to cover my changes, and all previous tests pass.
• [ x] I have checked all compat numbers if they need updating (FPGA compat, MPM compat, noc_shell, specific RFNoC block, ...)

[github-actions[bot]]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[icyveins7]

I have read the CLA Document and I hereby sign the CLA

[icyveins7]

recheck

[chockey]

For what it's worth, I have the same compile problem and had applied this exact same fix to my local tree to fix it before finding this PR

[mbr0wn]

I'm putting this into our internal review/CI queue.

[mbr0wn]

Thanks for this fix!