avr: use the correct RAM start address

Previously, the RAM was set to start at address 0. This is incorrect: on
AVR, the first few addresses are taken up by memory-mapped I/O. The
reason this didn't lead to problems (yet) was because the stack was
usually big enough to avoid real problems.

targets/avr.ld

@@ -1,8 +1,8 @@
 
 MEMORY
 {
-    FLASH_TEXT (rw) : ORIGIN = 0, LENGTH = __flash_size - _bootloader_size
-    RAM (xrw)       : ORIGIN = 0, LENGTH = __ram_size
+    FLASH_TEXT (rw) : ORIGIN = 0,           LENGTH = __flash_size - _bootloader_size
+    RAM (xrw)       : ORIGIN = __ram_start, LENGTH = __ram_size
 }
 
 SECTIONS