recipetool / devtool: set a fixed SRCREV by default when fetching from git

If fetching source from a git repository, typically within OpenEmbedded
we encourage setting SRCREV to a fixed revision, so change to do that by
default and add a  -a/--autorev option to use "${AUTOREV}" instead.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>