Correctly set version build flags. (#82)

Author: Gerrit91

Makefile

@@ -1,8 +1,8 @@
-.ONESHELL:
 SHA := $(shell git rev-parse --short=8 HEAD)
 GITVERSION := $(shell git describe --long --all)
-BUILDDATE := $(shell date -Iseconds)
-VERSION := $(or ${VERSION},devel)
+# gnu date format iso-8601 is parsable with Go RFC3339
+BUILDDATE := $(shell date --iso-8601=seconds)
+VERSION := $(or ${VERSION},$(shell git describe --tags --exact-match 2> /dev/null || git symbolic-ref -q --short HEAD || git rev-parse --short HEAD))
 
 all: test metal-bmc