upgrades: Allow backports from src:freedombox
This allows any binary package in src:freedombox to be upgraded from backports, so that packaged manuals are included.
Thanks for Frank Woodward for finding the root cause and solution.
Tests:
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Build a FreedomBox bullseye image with backports disabled.
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During setup, enable backports, and update packages. freedombox package from bullseye-backports is installed, but freedombox-doc-* packages are from bullseye.
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Install freedombox package with the fix, and update packages. freedombox-doc-* packages from bullseye-backports are installed.
Closes #2106 (closed).
Signed-off-by: James Valleroy jvalleroy@mailbox.org