avr: make linker script generic

targets/arduino.json

@@ -2,7 +2,16 @@
 	"llvm-target": "avr-atmel-none",
 	"build-tags": ["arduino", "atmega328p", "atmega", "avr5", "avr", "js", "wasm"],
 	"linker": "avr-gcc",
-	"pre-link-args": ["-nostartfiles", "-mmcu=avr5", "-T", "targets/avr.ld", "-Wl,--gc-sections", "targets/avr.S", "src/device/avr/atmega328p.s"],
+	"pre-link-args": [
+		"-nostartfiles",
+		"-mmcu=avr5",
+		"-Wl,--defsym=_bootloader_size=512",
+		"-T", "src/device/avr/atmega328p.ld",
+		"-T", "targets/avr.ld",
+		"-Wl,--gc-sections",
+		"targets/avr.S",
+		"src/device/avr/atmega328p.s"
+	],
 	"objcopy": "avr-objcopy",
 	"flash": "avrdude -c arduino -p atmega328p -P {port} -U flash:w:{hex}"
 }

targets/avr.ld

@@ -1,11 +1,10 @@
 
 MEMORY
 {
-    FLASH_TEXT (rw) : ORIGIN = 0, LENGTH = 32K - _bootloader_size
-    RAM (xrw)       : ORIGIN = 0, LENGTH = 2K
+    FLASH_TEXT (rw) : ORIGIN = 0, LENGTH = __flash_size - _bootloader_size
+    RAM (xrw)       : ORIGIN = 0, LENGTH = __ram_size
 }
 
-_bootloader_size = 512;
 _stack_size = 512;
 
 SECTIONS