bootm: Add tests for fixup_silent_linux()

This function currently has no tests. Export it so that we can implement
a simple test on sandbox. Use IS_ENABLED() to remove the unused code,
instead #ifdef.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/bootm.h

@@ -85,4 +85,7 @@ void arch_preboot_os(void);
  */
 void board_preboot_os(void);
 
+/* Adjust the 'bootargs' to ensure that Linux boots silently, if required */
+void fixup_silent_linux(void);
+
 #endif