classes/buildcfg: Move git/layer revision code into new OE module buildcfg

There is a load of duplicated git/layer/revision code which makes
most sesne as a python library, not bbclass code. Start to refactor as such.

(From OE-Core rev: 439cdf8a1e52fd2c4dc81dc40ce7e6af282ce7ac)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/metadata_scm.bbclass

@@ -1,44 +1,5 @@
-def base_detect_revision(d):
-    path = base_get_scmbasepath(d)
-    return base_get_metadata_git_revision(path, d)
 
-def base_detect_branch(d):
-    path = base_get_scmbasepath(d)
-    return base_get_metadata_git_branch(path, d)
-
-def base_get_scmbasepath(d):
-    return os.path.join(d.getVar('COREBASE'), 'meta')
-
-def base_get_metadata_svn_revision(path, d):
-    # This only works with older subversion. For newer versions 
-    # this function will need to be fixed by someone interested
-    revision = "<unknown>"
-    try:
-        with open("%s/.svn/entries" % path) as f:
-            revision = f.readlines()[3].strip()
-    except (IOError, IndexError):
-        pass
-    return revision
-
-def base_get_metadata_git_branch(path, d):
-    import bb.process
-
-    try:
-        rev, _ = bb.process.run('git rev-parse --abbrev-ref HEAD', cwd=path)
-    except bb.process.ExecutionError:
-        rev = '<unknown>'
-    return rev.strip()
-
-def base_get_metadata_git_revision(path, d):
-    import bb.process
-
-    try:
-        rev, _ = bb.process.run('git rev-parse HEAD', cwd=path)
-    except bb.process.ExecutionError:
-        rev = '<unknown>'
-    return rev.strip()
-
-METADATA_BRANCH := "${@base_detect_branch(d)}"
+METADATA_BRANCH := "${@oe.buildcfg.detect_branch(d)}"
 METADATA_BRANCH[vardepvalue] = "${METADATA_BRANCH}"
-METADATA_REVISION := "${@base_detect_revision(d)}"
+METADATA_REVISION := "${@oe.buildcfg.detect_revision(d)}"
 METADATA_REVISION[vardepvalue] = "${METADATA_REVISION}"