Always generate MFC Blackbox filelist

[abejgonzalez]

If a design has no blackboxes it might not generate a black-box filelist (MFC...BB_FILELIST). This breaks the flow (since a sed needs it downstream). Instead always generate it.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

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