x86/geode: fixup FEATURE inheritance

In the geode subtarget all default x86 features were overwritten via :=
instead of extending them via +=.

This patch fixes the inheritance and thereby the compilation of
x86/geode target.

Compile tested x86/geode.

Signed-off-by: Paul Spooren <mail@aparcar.org>

target/linux/x86/geode/target.mk

@@ -1,5 +1,5 @@
 BOARDNAME:=AMD Geode based systems
-FEATURES:=pci usb gpio
+FEATURES += pci usb gpio
 DEFAULT_PACKAGES += \
 			kmod-crypto-hw-geode kmod-crypto-cbc \
 			kmod-ath5k kmod-ath9k \