Update .gitmodules with submodule branch information

Changelog-entry: Update .gitmodules with submodule branch information
Signed-off-by: Kyle Harding <kyle@balena.io>

Author: klutchell

.gitmodules

@@ -1,9 +1,12 @@
 [submodule "layers/meta-balena"]
 	path = layers/meta-balena
 	url = https://github.com/balena-os/meta-balena.git
+	branch = master
 [submodule "balena-yocto-scripts"]
 	path = balena-yocto-scripts
 	url = https://github.com/balena-os/balena-yocto-scripts.git
+	branch = master
 [submodule "contracts"]
 	path = contracts
 	url = https://github.com/balena-io/contracts.git
+	branch = master