addons/get_kvers.sh: fix issue with CONFIG_LOCALVERSION_AUTO disabled

Modified kernels with CONFIG_LOCALVERSION_AUTO disabled produce a short
version string with the form "<MAJOR>.<MINOR>.<SUBVERS>+". This case
was not handled correctly by the version parser.

Signed-off-by: Erick Reyes <erickreyes@google.com>
diff --git a/addons/get_kvers.sh b/addons/get_kvers.sh
index 3a7a6fe..3d18e32 100755
--- a/addons/get_kvers.sh
+++ b/addons/get_kvers.sh
@@ -4,7 +4,7 @@
 
 MAJOR=$(echo $kvers | awk -F. '{ print $1 }')
 MINOR=$(echo $kvers | awk -F. '{ print $2 }')
-SUBVR=$(echo $kvers | awk -F. '{ print $3 }' | awk -F- '{ print $1 }')
+SUBVR=$(echo $kvers | awk -F. '{ print $3 }' | awk -F- '{ print $1 }' | sed 's/[^0-9]*//g')
 
 maj_num=$(($MAJOR * 65536))
 min_num=$(($MINOR * 256))