move all target in Makefile.defs to Makefile
remove git describe in koreader-base

the git-rev file is used to track distrubtion version for the whole
koreader, not just koreader-base, so it makes more sense to move it
to koreader and kindlepdfviewer.
udpate README
Merge pull request #1 from houqp/make-fix

move all target in Makefile.defs to Makefile