liteeth should be tolerant to truncated or garbled preambles

[jordens]

• the preamble is there to synchronize the clock. corruption is benign, especially at the beginning
• the frame start is signalled by the SFD, not by the preamble

presumably root cause for https://github.com/m-labs/artiq/issues/837#issuecomment-347281126

[whitequark]

@jordens I have an in-progress patch for this (it doesn't work yet).