avr/atmega: Resolve memory corruption preventing boot

Resolve memory corruption issue where the memory values to be loaded
from flash to sram at boot contained incorrect values. Also added a
KEEP() guard around `.vectors` to prevent debug stripping from removing
them, as suggested by KR.

Signed-off-by: Matteo Golin <matteo.golin@gmail.com>

boards/avr/atmega/arduino-mega2560/scripts/flash.ld

@@ -112,7 +112,7 @@ SECTIONS
     .text            :
     {
         _stext = . ;
-        *(.vectors)
+        KEEP(*(.vectors))
         *(.init)
         *(.handlers)
         *(.progmem .progmem.*)
@@ -122,8 +122,6 @@ SECTIONS
         _etext = . ;
     } > flash
 
-    _eronly = ABSOLUTE(.);
-
     .data            :
     {
         _sdata = ABSOLUTE(.);
@@ -134,6 +132,8 @@ SECTIONS
         _edata = ABSOLUTE(.);
     } > sram AT > flash
 
+    _eronly = LOADADDR(.data);
+
     .bss            :
     {
         _sbss = ABSOLUTE(.);