Currently error-report doesn't manage Unicode because
the files are opened with the default codec.
This patch changes the codec of the files to UTF-8,
this way the reports will include Unicode characters.
This is useful for the qemu output when doing the
testimage task.
[YOCTO #8225]
Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
