portability: use py script instead of 'date -d @1234'

On FreeBSD, the 'date' command's -d option has a completely different meaning.
Instead, use a small python script to do the date format conversion, which
should be more portable.

As a side effect, we now also use UTC instead of the build server's timezone,
which may be considered a more international choice.

Add build/unix-time-to-fmt.py, call in build/Makefile.asciidoc.inc.

Change-Id: I91a40656184f553ee375216d8ba5c7788fe9990d

build/Makefile.asciidoc.inc

@@ -1,7 +1,7 @@
 BUILDDIR = $(TOPDIR)/build
 
 GIT_VERSION := $(shell git describe --abbrev=4 --dirty --always --tags)
-GIT_DATE := $(shell date -d @`git log -n 1 "--pretty=%at" ../.` "+%Y-%b-%e")
+GIT_DATE := $(shell $(TOPDIR)/build/unix-time-to-fmt.py `git log -n 1 "--pretty=%at" ../.`)
 
 # prepend the document name with the version numbe suffix
 #DOCS_VER = $(foreach P, $(ASCIIDOCS), $(P)-v$(shell xmllint --recover --xpath "//revnumber[position()=last()]/text()" $(P)-docinfo.xml 2>/dev/null))

build/unix-time-to-fmt.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+"""
+Usage:
+
+  unix-time-to-fmt.py 1234567 [%Y-%m-%d[...]]
+
+Convert unix timestamp to a string of the given format in UTC, according to
+  https://docs.python.org/2/library/time.html
+Default is '%Y-%b-%d' --> 2016-Jan-01
+"""
+
+import sys, time
+
+fmt = '%Y-%b-%d'
+if len(sys.argv) > 2:
+  fmt = sys.argv[2]
+
+print(time.strftime(fmt, time.gmtime(float(sys.argv[1]))))