Add hostboot-binaries package env version/site overrides

Support environment override of HOSTBOOT_BINARIES_VERSION 
Support environment override of HOSTBOOT_BINARIES_SITE
diff --git a/openpower/package/hostboot-binaries/hostboot_binaries.mk b/openpower/package/hostboot-binaries/hostboot_binaries.mk
index 332110c..0e3f087 100644
--- a/openpower/package/hostboot-binaries/hostboot_binaries.mk
+++ b/openpower/package/hostboot-binaries/hostboot_binaries.mk
@@ -4,8 +4,8 @@
 #
 ################################################################################
 
-HOSTBOOT_BINARIES_VERSION = da5ca565422d157e7225d3f20c7ff06f63610f49
-HOSTBOOT_BINARIES_SITE = $(call github,open-power,hostboot-binaries,$(HOSTBOOT_BINARIES_VERSION))
+HOSTBOOT_BINARIES_VERSION ?= da5ca565422d157e7225d3f20c7ff06f63610f49
+HOSTBOOT_BINARIES_SITE ?= $(call github,open-power,hostboot-binaries,$(HOSTBOOT_BINARIES_VERSION))
 HOSTBOOT_BINARIES_LICENSE = Apache-2.0
 
 HOSTBOOT_BINARIES_INSTALL_IMAGES = YES