arc770: mark as source-only

Mark target arc770 as source-only as it has package compile issues
(e.g. iproute2) due to the usage of uClibc.
As a prerequisite to be included in future releases the arc770 target
needs to switch either to glibc or musl.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

target/linux/arc770/generic/target.mk

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