ipq40xx: stop chromium sub-target builds on the buildbots

the buildbots are having troubles with the image.
They seem to get "Killed" at the last step of the KERNEL rule:

|/cros-vbutil -k zImage.itb.vboot -c "root=PARTUUID=%U/PARTNROFF=1" -o zImage.itb.vboot.new
|make[4]: *** [Makefile:18: zImage.itb.vboot] Killed

Since the Google Wifi (Gale) is currently the only target in
this sub-target. So this means that subtarget has to be disabled
from the time being to not be picked up by the builders.

For people wanting to checkout out OpenWrt on the Google Wifi:
please compile it locally.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

target/linux/ipq40xx/chromium/target.mk

@@ -1,2 +1,2 @@
 BOARDNAME:=Google Chromium
-FEATURES += emmc boot-part rootfs-part
+FEATURES += emmc boot-part rootfs-part source-only