Tools: allow size_compare_branches.py to test boards with modified hwdefs

[peterbarker]

With just this commit:

ValueError: No modified boards found between pr/size-compare-branches-test-modifies-boards and master

Modifying libraries/AP_HAL_ChibiOS/hwdef/CubeOrange/hwdef.inc in a test commit on top:

pbarker@crun:~/rc/ardupilot-claude(pr-claude/size-compare-branches-test-modifies-boards)$ ./Tools/scripts/size_compare_branches.py --modified-boards
SCB: Running (git symbolic-ref --short HEAD) in (.)
SCB-GIT: pr-claude/size-compare-branches-test-modifies-boards
SCB: Running (git merge-base pr-claude/size-compare-branches-test-modifies-boards master) in (.)
SCB-GIT: 92d46eca62c3e380af26737f74a6fa914739a5ea
SCB: Running (git diff --name-only 92d46eca62c3e380af26737f74a6fa914739a5ea pr-claude/size-compare-branches-test-modifies-boards) in (.)
SCB: Modified hwdef: libraries/AP_HAL_ChibiOS/hwdef/CubeOrange/hwdef.inc
SCB: Board CubeOrange-periph-heavy uses modified hwdef
SCB: Board CubeOrange-bdshot uses modified hwdef
SCB: Board CubeOrangePlus-bdshot uses modified hwdef
SCB: Board CubeOrange-ODID uses modified hwdef
SCB: Board CubeOrange-periph uses modified hwdef
SCB: Board CubeOrange-joey uses modified hwdef
SCB: Board CubeOrangePlus-ODID uses modified hwdef
SCB: Board CubeOrange-SimOnHardWare uses modified hwdef
SCB: Board CubeOrangePlus-SimOnHardWare uses modified hwdef
SCB: Board CubeOrange uses modified hwdef
SCB: Board CubeOrangePlus uses modified hwdef
SCB: Running (git merge-base pr-claude/size-compare-branches-test-modifies-boards master) in (.)
SCB-GIT: 92d46eca62c3e380af26737f74a6fa914739a5ea
SCB: Using merge base (92d46eca62c3e380af26737f74a6fa914739a5ea)
.
.
.
SCB: Running (rsync -ap build/ /tmp/tmpe56a2y80/out-branch-CubeOrangePlus-SimOnHardWare) in (.)
Board,plane
CubeOrange,0
CubeOrange-ODID,*
CubeOrange-bdshot,*
CubeOrange-joey,*
CubeOrangePlus,*
CubeOrangePlus-ODID,*
CubeOrangePlus-bdshot,*

pbarker@crun:~/rc/ardupilot-claude(pr-claude/size-compare-branches-test-modifies-boards)$