logger: flush remaining data from stdio writers on file release

logger: flush remaining data from stdio writers on file release

Unlike logs from regular writers, logs from stdio writers are not
contained in a single write_iter call: if the last character written in
the call is not a newline, some of the input remains in the buffer for
the next write_iter call. This means that some data might remain in the
buffer during release, and such data has to be flushed; otherwise, it
would be lost.

Change-Id: I13c9dc020967fe8ef3fbcd52d45f85b8725d3c2c
Signed-off-by: Mateusz Majewski <m.majewski2@samsung.com>