New design:
* capture output to a vstr
* compare the complete vstr to boot_out.txt
* rewrite if not a complete match
This is resilient against future changes to the automatic
text written to boot_out.txt.
This also fixes rewriting boot_out.txt in the case where
boot.py prints something.
Perhaps it also saves a bit of code space. Some tricks:
* no need to close a file in read mode
* no need to switch on/off USB write access, going down to the
oofatfs layer doesn't check it anyway
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