Thanks to sherb38607 for an empty flag design. Check out his replacements here: http://www.nexusmods.com/fallout4/mods/314/?
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Description:
This is a flag replacement mod which replaces the pre-war US-flag with the NCR flag known from FA:NV
New: Also added a dirty version of the flag
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Installation:
- Extract the textures folder to the fallout 4 data folder:
...\steamapps\common\Fallout 4\Data

- Change this line in the Fallout4.ini...
sResourceDataDirsFinal=STRINGS\

- To this, if you haven't already done so...
sResourceDataDirsFinal=STRINGS\, TEXTURES\

- The file is located in your documents folder here:
...\Documents\My Games\Fallout4

- Save the ini file 

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New: I added an other mod with some more flags here:
http://www.nexusmods.com/fallout4/mods/1335/?