recipetool: Set master branch only as fallback

The commit 'meta/scripts: Manual git url branch additions (dc53fe75cc)'
sets the branch= parameter too early to master and thereby breaks the
-B/--srcbranch option.

ERROR: branch= parameter and -B/--srcbranch option cannot both be specified - use one or the other

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>