arm: async echo app

arm/app.ld

@@ -7,9 +7,12 @@ SECTIONS
 {
     .text :
     {
+        _text_begin = .;
         KEEP(*(.app_init))
 
         *(.text*)
+        _text_end = .;
+
         *(.rodata*)
     } > ICTM
 
@@ -30,5 +33,8 @@ SECTIONS
         __exidx_end = .;
     } > ICTM
 
+    _heap_begin = .;
+
     _initial_stack_pointer = 16384;
+    _heap_end = _initial_stack_pointer - 1024;
 }