console: update silent tag after env load

Update the "silent" property with the variable "silent" get from
saved environment, it solves the issue when:
- CONFIG_SILENT_CONSOLE and CONFIG_SYS_CONSOLE_IS_IN_ENV are activated
- silent is not defined in default environment
- silent is requested in saved environment with:
  > env set silent 1; env save

On next reboot the console is not disabled as expected after relocation
and the environment load from flash (the callback is not called when
the INSERT is requested in the created hash table)

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