build: add configure option to not build BMC module

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I8b57a89f03c6fba2c07bc1069a9ef758f34ad4d4
diff --git a/Makefile.am b/Makefile.am
index 0626773..8743f70 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -36,7 +36,11 @@
 
 ACLOCAL_AMFLAGS = -I m4
 
-SUBDIRS = bmc
+SUBDIRS =
+
+if BUILD_BMC_HANDLER
+SUBDIRS += bmc
+endif
 
 if BUILD_HOST_TOOL
 SUBDIRS += tools