swupdate: introduce var to build from different branch

Add a SRCBRANCH in cases it is required to get a maintenance branch.

Signed-off-by: Stefano Babic <stefano.babic@swupdate.org>

recipes-support/swupdate/swupdate.inc

@@ -17,7 +17,8 @@ LIC_FILES_CHKSUM = "file://LICENSES/GPL-2.0-only.txt;md5=4ee23c52855c222cba72583
 
 inherit cml1 update-rc.d systemd pkgconfig
 
-SRC_URI = "git://github.com/sbabic/swupdate.git;protocol=https;branch=master \
+SRCBRANCH ?= "master"
+SRC_URI = "git://github.com/sbabic/swupdate.git;protocol=https;branch=${SRCBRANCH} \
     file://defconfig \
     file://swupdate \
     file://swupdate.sh \