Add hostboot package env version/site overrides

Support environment override of HOSTBOOT_VERSION 
Support environment override of HOSTBOOT_SITE
diff --git a/openpower/package/hostboot/hostboot.mk b/openpower/package/hostboot/hostboot.mk
index ea219d5..71140c9 100644
--- a/openpower/package/hostboot/hostboot.mk
+++ b/openpower/package/hostboot/hostboot.mk
@@ -4,8 +4,8 @@
 #
 ################################################################################
 
-HOSTBOOT_VERSION = 1300dc02979646be65a39097f86a559cf68bcfb7
-HOSTBOOT_SITE = $(call github,open-power,hostboot,$(HOSTBOOT_VERSION))
+HOSTBOOT_VERSION ?= 1300dc02979646be65a39097f86a559cf68bcfb7
+HOSTBOOT_SITE ?= $(call github,open-power,hostboot,$(HOSTBOOT_VERSION))
 HOSTBOOT_LICENSE = Apache-2.0
 HOSTBOOT_DEPENDENCIES = host-binutils