console: execute flush on uart when silent is removed

Avoid to flush buffer when silent console is activated as the
console can be reactivate later, after relocation, when the env will
be updated with the saved one.

Solve issue (missing beginning of U-Boot trace) when:
- CONFIG_SILENT_CONSOLE is activated
- silent=1 is defined in default environment (CONFIG_EXTRA_ENV_SETTINGS)
- silent is removed in saved environment with:
      > env delete silent; env save

Only functional when PRE_CONSOLE_BUFFER is activated.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>