osbuilder: Fix Makefile
Let the DISTRO variable to be set from outside,
allowing "sudo -E DISTRO=<ANY> make clean" to delete the correct files.
@@ -9,7 +9,7 @@ ROOTFS_BUILDER := $(MK_DIR)/rootfs-builder/rootfs.sh
INITRD_BUILDER := $(MK_DIR)/initrd-builder/initrd_builder.sh
IMAGE_BUILDER := $(MK_DIR)/image-builder/image_builder.sh
-DISTRO := centos
+DISTRO ?= centos
BUILD_METHOD := distro
BUILD_METHOD_LIST := distro dracut
AGENT_INIT ?= no
