arc770: remove source-only

In commit 8b9cdebc9c ("arc770: mark as source-only") arc770 was marked
as source-only because of iproute2 compile issues.
With uClibc-ng version 1.0.30 issues with iproute2 were fixed.
Lets remove "source-only" for arc770 as soon as uClibc-ng version
will be updated to 1.0.30. Patch for uClibc-ng is here:
https://patchwork.ozlabs.org/patch/917547/

Signed-off-by: Evgeniy Didin <Evgeniy.Didin@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: John Crispin <john@phrozen.org>

target/linux/arc770/generic/target.mk

@@ -1,5 +1,5 @@
 BOARDNAME:=Generic
-FEATURES += ramdisk usb ext4 source-only
+FEATURES += ramdisk usb ext4 
 
 define Target/Description
 	Build firmware images for generic ARC 770D based boards.