avr: make stack size configurable

targets/arduino.json

@@ -6,6 +6,7 @@
 		"-nostartfiles",
 		"-mmcu=avr5",
 		"-Wl,--defsym=_bootloader_size=512",
+		"-Wl,--defsym=_stack_size=512",
 		"-T", "src/device/avr/atmega328p.ld",
 		"-T", "targets/avr.ld",
 		"-Wl,--gc-sections",

targets/avr.ld

@@ -5,8 +5,6 @@ MEMORY
     RAM (xrw)       : ORIGIN = 0, LENGTH = __ram_size
 }
 
-_stack_size = 512;
-
 SECTIONS
 {
     .text :