configure.in: change check for 'VERSION' file:
- first check if this file exist and use the content from there
- if it not exist check with 'git log'
- if this fails set to 'Unknown'

This patch also shows the GIT_REV in the configure summary message. To display This message on top of the messages we move the whole block a bit above
Merge pull request #2193 from OpenELEC/git-version-changes

configure.in: change check for 'VERSION' file: