[OS-BUILD PATCHv3 0/3] process_configs.sh: Fix syntax flagged by shellcheck

[GitLab Bridge on behalf of bcrocker]

From: bcrocker on gitlab.com

process_configs.sh: Fix syntax flagged by shellcheck\

• SC2006: Use $(...) notation instead of legacy backticked `...`.\
• SC2034: <variable> appears unused. Verify use (or export if used
externally).\
• SC2045: Iterating over ls output is fragile. Use globs.\
• SC2046: Quote this to prevent word splitting.\
• SC2086: Double quote to prevent globbing and word splitting.\
• SC2231: Quote expansions in this for-loop glob to prevent
wordsplitting, e.g. "$dir"/*.txt .\

Ignore:\
• SC2164: Use 'cd ... || exit' or 'cd ... || return' in case cd fails.\