orion: Make sub-targets augment FEATURES, not re-define it

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

target/linux/orion/generic/target.mk

@@ -6,7 +6,7 @@
 #
 
 BOARDNAME:=Generic
-FEATURES:=squashfs
+FEATURES+=squashfs
 
 define Target/Description
 	Build firmware images for Marvell Orion based boards that boot from internal flash.

target/linux/orion/harddisk/target.mk

@@ -6,7 +6,7 @@
 #
 
 BOARDNAME:=Internal Hard-Disk
-FEATURES:=targz
+FEATURES+=targz
 
 define Target/Description
 	Build firmware images for Marvell Orion based boards that boot directly from internal disk storage.