librem5-flash-image: Add option to set udev rules

This avoids downloading / flashing with root permissions and
extending the docs even further.

We can switch to `uuu -udev > ...` once the new uuu is more
widespread.

Closes: #35
3 jobs for pureos/byzantium in 2 minutes and 56 seconds (queued for 2 seconds)
Status Job ID Name Coverage
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passed #330454
librem5
build:deb

00:01:09

 
  Test
passed #330455
librem5
test:download:deb

00:01:41

passed #330456
librem5
test:download:pip

00:01:47