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/test/Makefile b/test/Makefile
index 8296734..d4323f9 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -5,6 +5,7 @@
 ifneq ($(CONFIG_SANDBOX),)
 obj-$(CONFIG_$(SPL_)CMDLINE) += bloblist.o
 endif
+obj-$(CONFIG_$(SPL_)CMDLINE) += bootm.o
 obj-$(CONFIG_$(SPL_)CMDLINE) += cmd/
 obj-$(CONFIG_$(SPL_)CMDLINE) += cmd_ut.o
 obj-$(CONFIG_$(SPL_)CMDLINE) += command_ut.o