basecmd: Move low-level alloc code into basecmd.c

Implement new dynmem_start() and dynmem_end() functions instead of
alloc_chunk() and alloc_chunks() in the board code.  This simplifies
the board code.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/sam3x8e/main.c

@@ -4,7 +4,6 @@
 //
 // This file may be distributed under the terms of the GNU GPLv3 license.
 
-#include "board/misc.h" // alloc_maxsize
 #include "command.h" // DECL_CONSTANT
 #include "sam3x8e.h" // WDT
 #include "sched.h" // sched_main
@@ -43,16 +42,6 @@ command_reset(uint32_t *args)
 }
 DECL_COMMAND_FLAGS(command_reset, HF_IN_SHUTDOWN, "reset");
 
-void * __visible
-_sbrk(int nbytes)
-{
-    extern char _end;
-    static void *heap_ptr = (void *)&_end;
-    void *pos = heap_ptr;
-    heap_ptr = pos + nbytes;
-    return pos;
-}
-
 // Main entry point
 int
 main(void)