Allow .bbappend files to overwrite SRCREV

For example, when putting the following content into a swupdate_git.bbappend,
the most recent commit in master as of 2018-09-05 gets used:

SRCREV = 'd8069ae82853c2dc6b8400a29ec51b1b556e4134'

Signed-off-by: Reto Schneider <code@reto-schneider.ch>
Acked-by: Stefano Babic <sbabic@denx.de>

recipes-support/swupdate/swupdate_git.bb

@@ -3,4 +3,4 @@ require swupdate_tools.inc
 
 DEFAULT_PREFERENCE = "-1"
 
-SRCREV = '8a2ff356c3d93bb4f681a511e6b42ed64dcf6409'
+SRCREV ?= '8a2ff356c3d93bb4f681a511e6b42ed64dcf6409'