[projmgr] Add support for image-only solutions

[grasci-arm]

Address #2100

[github-actions]

Test Results

    3 files     21 suites   12m 6s ⏱️
  398 tests   398 ✅ 0 💤 0 ❌
1 194 runs  1 194 ✅ 0 💤 0 ❌

Results for commit 21f62ef.

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 8 lines in your changes missing coverage. Please review.

Project coverage is 66.05%. Comparing base (d8089a3) to head (21f62ef).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
tools/projmgr/src/ProjMgrCbuildIdx.cpp	66.66%	0 Missing and 3 partials ⚠️
tools/projmgr/src/ProjMgrCbuild.cpp	92.59%	0 Missing and 2 partials ⚠️
tools/projmgr/src/ProjMgrWorker.cpp	92.00%	0 Missing and 2 partials ⚠️
tools/projmgr/test/src/ProjMgrUnitTests.cpp	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2138      +/-   ##
==========================================
+ Coverage   66.03%   66.05%   +0.02%     
==========================================
  Files         181      181              
  Lines       37243    37281      +38     
  Branches    21670    21693      +23     
==========================================
+ Hits        24592    24627      +35     
- Misses       8203     8204       +1     
- Partials     4448     4450       +2     

Flag	Coverage Δ
projmgr-cov	80.50% <90.47%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
tools/projmgr/include/ProjMgrWorker.h	77.27% <ø> (ø)
tools/projmgr/src/ProjMgr.cpp	81.61% <100.00%> (+0.15%)	⬆️
tools/projmgr/src/ProjMgrYamlParser.cpp	83.40% <ø> (-0.06%)	⬇️
...projmgr/test/src/ProjMgrSchemaCheckerUnitTests.cpp	79.51% <100.00%> (ø)
tools/projmgr/test/src/ProjMgrUnitTests.cpp	80.49% <92.85%> (+0.03%)	⬆️
tools/projmgr/src/ProjMgrCbuild.cpp	94.11% <92.59%> (+0.05%)	⬆️
tools/projmgr/src/ProjMgrWorker.cpp	88.12% <92.00%> (-0.05%)	⬇️
tools/projmgr/src/ProjMgrCbuildIdx.cpp	92.92% <66.66%> (-0.89%)	⬇️

... and 2 files with indirect coverage changes

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