linux: Enable fbcon deferred takeover when possible
linux: Enable fbcon deferred takeover when possible

This config value ensures that when booting through e.g. UEFI, the
existing framebuffer contents stay put until the first character is
printed. As the default NixOS stage-1 immediately outputs a welcome
message on init, this does not impact it, but it will allow for a cleaner boot when
configured as such.

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