Allow compiling common/bootm.c on with HOSTCC

We want to use some of the functionality in this file, so make it
build on the host.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/Makefile b/tools/Makefile
index 0088c1a..949b6c6 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -66,6 +66,7 @@
 dumpimage-mkimage-objs := aisimage.o \
 			atmelimage.o \
 			$(FIT_SIG_OBJS-y) \
+			common/bootm.o \
 			lib/crc32.o \
 			default_image.o \
 			lib/fdtdec.o \