build-appliance-image: Allow SRCREV to be overriden

This will allow use to automagically set the SRCREV for builds on the
autobuilder. It will still require manual updating for releases.

(From OE-Core rev: 1b4781e5c6eee234fcf57dd53d5167b31d81a482)

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-core/images/build-appliance-image.bb

@@ -20,7 +20,7 @@ IMAGE_FSTYPES = "vmdk"
 
 inherit core-image
 
-SRCREV = "73cdebf60df225ee10f2eb215935be3b61e1b831"
+SRCREV ?= "73cdebf60df225ee10f2eb215935be3b61e1b831"
 SRC_URI = "git://git.yoctoproject.org/poky;protocol=git \
            file://Yocto_Build_Appliance.vmx \
            file://Yocto_Build_Appliance.vmxf \