ath79: make target source-only

In order to prevent build of images for this release as ath79 is going
to deprecate ar71xx in the next release.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ath79/Makefile

@@ -6,7 +6,7 @@ BOARDNAME:=Atheros ATH79 (DTS)
 CPU_TYPE:=24kc
 SUBTARGETS:=generic nand tiny
 
-FEATURES:=ramdisk
+FEATURES:=ramdisk source-only
 
 KERNEL_PATCHVER:=4.14