feat(Makefile): allow setting dracut version via environment variables

To ease packaging dracut for Debian/Ubuntu, please support setting the
dracut main/full version via environment variables:

```
$ DRACUT_MAIN_VERSION=007
$ export DRACUT_FULL_VERSION=007-bond
$ make dracut-version.sh
$ grep VERSION dracut-version.sh
DRACUT_VERSION=007-bond
```

Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>

Makefile

@@ -1,10 +1,10 @@
 -include dracut-version.sh
 
-DRACUT_MAIN_VERSION := $(shell env GIT_CEILING_DIRECTORIES=$(CURDIR)/.. git describe --abbrev=0 --tags --always 2>/dev/null || :)
+DRACUT_MAIN_VERSION ?= $(shell env GIT_CEILING_DIRECTORIES=$(CURDIR)/.. git describe --abbrev=0 --tags --always 2>/dev/null || :)
 ifeq ($(DRACUT_MAIN_VERSION),)
 DRACUT_MAIN_VERSION = $(DRACUT_VERSION)
 endif
-DRACUT_FULL_VERSION := $(shell env GIT_CEILING_DIRECTORIES=$(CURDIR)/.. git describe --tags --always 2>/dev/null || :)
+DRACUT_FULL_VERSION ?= $(shell env GIT_CEILING_DIRECTORIES=$(CURDIR)/.. git describe --tags --always 2>/dev/null || :)
 ifeq ($(DRACUT_FULL_VERSION),)
 DRACUT_FULL_VERSION = $(DRACUT_VERSION)
 endif