flash does not work if gnu sed is installed

[neben]

If GNU sed is installed, flash fails on MacOS:

Set hostname=foo
sed: can't read : No such file or directory

This is a regression, the original issue is #49. Either sed should be called with its full path, i.e. /usr/bin/sed, or the workaround should be dropped altogether. On my MacOS /usr/bin/sed -i works without the extra empty string, i.e. "", why was it needed in the first place?