Bound Elbert local MAC address read

[Rohithmatham12]

Fixes #126.

This bounds the local MAC address read in elbert_weutil:

• passes the destination buffer size into read_local_mac
• initializes the fallback value with snprintf
• limits the sysfs read to the destination buffer size
• checks the fscanf result and preserves the "NA" fallback on malformed input

Test plan:

git diff --check

I did not run a full OpenBMC image build locally because the Yocto/OpenBMC build environment and platform layers are not configured in this workspace.

[meta-codesync]

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