Add openPOWER version string support

See VERSION.readme for more info
diff --git a/openpower/package/hostboot/hostboot.mk b/openpower/package/hostboot/hostboot.mk
index d3b76fa..e388305 100644
--- a/openpower/package/hostboot/hostboot.mk
+++ b/openpower/package/hostboot/hostboot.mk
@@ -15,7 +15,8 @@
 
 HOSTBOOT_ENV_VARS=$(TARGET_MAKE_ENV) \
     CONFIG_FILE=$(BR2_EXTERNAL)/configs/hostboot/$(BR2_HOSTBOOT_CONFIG_FILE) \
-    OPENPOWER_BUILD=1 CROSS_PREFIX=$(TARGET_CROSS) HOST_PREFIX="" HOST_BINUTILS_DIR=$(HOST_BINUTILS_DIR)
+    OPENPOWER_BUILD=1 CROSS_PREFIX=$(TARGET_CROSS) HOST_PREFIX="" HOST_BINUTILS_DIR=$(HOST_BINUTILS_DIR) \
+    HOSTBOOT_VERSION=`cat $(HOSTBOOT_VERSION_FILE)`
 
 define HOSTBOOT_BUILD_CMDS
         $(HOSTBOOT_ENV_VARS) bash -c 'cd $(@D) && source ./env.bash && $(MAKE)'