[SAMR21] change stack allocation to use all free RAM (#2334)

Initially fixed size 8k was allocated for stack.
As result there was almost no free RAM. This
change creates stack on all free RAM.

Signed-off-by: Oleksandr Grytsov <[email protected]>

third_party/microchip/include/samr21x18a.ld

@@ -53,9 +53,6 @@ MEMORY
   ram      (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00008000
 }
 
-/* The stack size used by the application. NOTE: you need to adjust according to your application. */
-STACK_SIZE = DEFINED(STACK_SIZE) ? STACK_SIZE : DEFINED(__stack_size__) ? __stack_size__ : 0x2000;
-
 /* Section Definitions */
 SECTIONS
 {
@@ -151,15 +148,16 @@ SECTIONS
     } > ram
 
     /* stack section */
-    .stack (NOLOAD):
+    .stack_dummy (NOLOAD):
     {
-        . = ALIGN(8);
-        _sstack = .;
-        . = . + STACK_SIZE;
-        . = ALIGN(8);
-        _estack = .;
+        KEEP(*(.stack*))
     } > ram
 
+    _sstack = _ebss;
+    _estack = ORIGIN(ram) + LENGTH(ram);
+
+    ASSERT(_sstack < _estack, "region RAM overflowed with stack")
+
     . = ALIGN(4);
     _end = . ;
     end = _end;