Take into account bootloader size on UNO WiFI rev2

Fixes arduino/Arduino#8978

Author: facchinm

boards.txt

@@ -9,7 +9,7 @@ uno2018.pid.0=0x2145
 
 uno2018.upload.tool=avrdude
 uno2018.upload.protocol=xplainedmini_updi
-uno2018.upload.maximum_size=49152
+uno2018.upload.maximum_size=48640
 uno2018.upload.maximum_data_size=6144
 uno2018.upload.speed=115200
 uno2018.upload.extra_params=-Pusb