yocto’s git fetcher falls back to using ‘master’ for branchname if its not specified by the user in the recipe when defining SRC_URI, however, recently lot of opensource packages have started moving away from calling the default branch as master, therefore it is no longer a common default. Yocto now warns about such usage
Parsing recipes...WARNING: /home/jenkins/oe/world/yoe/sources/meta-freescale/recipes-bsp/mxsldr/mxsldr_git.bb: URL: git://git.denx.de/mxsldr.git does not set any branch parameter. The future default branch used by tools and repositories is uncertain and we will therefore soon require this is set in all git urls.
There is detailed discussion about this on oe-architecture mailing list email@example.com | Default branch names in git urls