samples: net: wpanusb: split cc2520 settings from prj.conf

When the CC2520-specific configs are removed from the prj.conf
file, the sample can be built for hardware that supports
802.15.4 natively w/o any changes to the prj.conf like so:
cmake -DBOARD=nrf52840_pca10056 ..
cmake -DBOARD=usb_kw24d512 ..

The CC2520-specific settings now live in overlay-cc2520.conf
which can be used like so when building for devices that can
use it:
cmake -DBOARD=quark_se_c1000_devboard \
  -DOVERLAY_CONFIG=overlay-cc2520.conf ..

Later, support for other optional HW like CC1200 and MCR20A
can be added in a similar way.

Signed-off-by: Michael Scott <mike@foundries.io>

samples/net/wpanusb/overlay-cc2520.conf

@@ -0,0 +1,2 @@
+CONFIG_IEEE802154_CC2520=y
+CONFIG_NET_APP_IEEE802154_DEV_NAME="cc2520"

samples/net/wpanusb/prj.conf

@@ -14,7 +14,6 @@ CONFIG_NET_BUF_DATA_SIZE=128
 
 CONFIG_IEEE802154=y
 CONFIG_IEEE802154_RAW_MODE=y
-CONFIG_IEEE802154_CC2520=y
 
 CONFIG_SYS_LOG=y
 CONFIG_SYS_LOG_IEEE802154_DRIVER_LEVEL=1
@@ -25,4 +24,3 @@ CONFIG_KERNEL_SHELL=y
 
 CONFIG_NET_APP_SETTINGS=y
 CONFIG_NET_APP_AUTO_INIT=n
-CONFIG_NET_APP_IEEE802154_DEV_NAME="cc2520"