sandbox: Split bootm code out into lib/bootm

Follow the convention of other architectures and move the platform
specific linux bootm code into sandbox/lib/bootm.c.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/lib/Makefile b/arch/sandbox/lib/Makefile
index 75b135c..96761e2 100644
--- a/arch/sandbox/lib/Makefile
+++ b/arch/sandbox/lib/Makefile
@@ -9,3 +9,4 @@
 
 obj-y	+= interrupts.o
 obj-$(CONFIG_PCI)	+= pci_io.o
+obj-$(CONFIG_CMD_BOOTM) += bootm.o