Add experimental support for ar71xx-nand target

Includes a profile for the NETGEAR WNDR3700v4 and WNDR4300v1

targets/ar71xx-nand/config

+CONFIG_TARGET_ar71xx=y
+CONFIG_TARGET_ar71xx_nand=y

targets/ar71xx-nand/kernel-vermagic

+41123e63bc43f61af09d064504300966

targets/ar71xx-nand/profiles.mk

+# List of hardware profiles
+
+## NETGEAR
+
+ifeq ($(BROKEN),1)
+# WNDR3700 v4, WNDR4300 v1
+$(eval $(call GluonProfile,WNDR4300))
+$(eval $(call GluonProfileFactorySuffix,WNDR4300,-ubi-factory,.img))
+$(eval $(call GluonProfileSysupgradeSuffix,WNDR4300,-squashfs-sysupgrade,.tar))
+$(eval $(call GluonModel,WNDR4300,wndr3700v4,netgear-wndr3700v4)) # BROKEN: untested
+$(eval $(call GluonModel,WNDR4300,wndr4300,netgear-wndr4300)) # BROKEN: untested
+endif

targets/targets.mk

 $(eval $(call GluonTarget,ar71xx,generic))
+$(eval $(call GluonTarget,ar71xx,nand))
 $(eval $(call GluonTarget,mpc85xx,generic))
 $(eval $(call GluonTarget,x86,generic))
 $(eval $(call GluonTarget,x86,kvm_guest))