qcacld-3.0: Add defconfigs for qca6490 and wcn7850

In preparation for upcoming Single Product Family (SPF) support add
per-target default configuration files for QCA6490 and WCN7850. In
addition add a default configuration file for the generic 'wlan'
target.

Change-Id: I0fb90c110bfa8b27a21327e2880642e1acb7853a
CRs-Fixed: 2890266

configs/qca6490_defconfig

@@ -0,0 +1 @@
+include $(WLAN_ROOT)/configs/default_defconfig

configs/wcn7850_defconfig

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+include $(WLAN_ROOT)/configs/default_defconfig

configs/wlan_defconfig

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+include $(WLAN_ROOT)/configs/default_defconfig