Shell: Remove unnecessary new lines at startup

When the shell start, it will print two lines. The reason for that
behavior is not documented. Remove that to make the code simpler and
shorter one line at a time.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>

Author: theob-pro

subsys/shell/shell.c

@@ -1446,9 +1446,10 @@ int shell_start(const struct shell *sh)
 		z_shell_vt100_color_set(sh, SHELL_NORMAL);
 	}
 
-	if (z_shell_strlen(sh->default_prompt) > 0) {
-		z_shell_raw_fprintf(sh->fprintf_ctx, "\n\n");
-	}
+	/* print new line before printing the prompt to clear the line
+	 * vt100 are not used here for compatibility reasons
+	 */
+	z_cursor_next_line_move(sh);
 	state_set(sh, SHELL_STATE_ACTIVE);
 
 	k_mutex_unlock(&sh->ctx->wr_mtx);