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>
diff --git a/arch/Kconfig b/arch/Kconfig
index 3aa99e0..accd4df 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -146,6 +146,7 @@
 	imply ACPI_PMC_SANDBOX
 	imply CMD_PMC
 	imply CMD_CLONE
+	imply SILENT_CONSOLE
 
 config SH
 	bool "SuperH architecture"