[nrfconnect] Fixed compilation with dfu overlay error. (#7418)

Due to the latest nRF Connect SDK version update few config names
related to Bluetooth changed. Device firmware upgrade overlay
that is used optionally was not aligned to those changes and now
target will not be build because of the compilation errors.

Changed Bluetooth configs names in DFU overlay to be compatible
with the new nRF Connect SDK version.

config/nrfconnect/app/overlay-dfu_support.conf

@@ -36,6 +36,5 @@ CONFIG_MCUMGR_SMP_BT_AUTHEN=n
 CONFIG_MCUMGR_BUF_COUNT=6
 
 # Increase BT MTU and RX buffer for big size DFU messages
-CONFIG_BT_L2CAP_TX_MTU=252
-CONFIG_BT_L2CAP_RX_MTU=252
-CONFIG_BT_RX_BUF_LEN=260
+CONFIG_BT_L2CAP_TX_MTU=260
+CONFIG_BT_BUF_ACL_RX_SIZE=264