Keep memory options together in makefile.

Moved the STACK_SIZE option up closer to the HEAP_TAGS and
WRITE_BUFFER_DISABLE options.
Switch heap start from __HeapBase to __end__.

I verified that these symbols point to the same address by diffing
the before and after main.bin output files.  This change caused no
change in the resulting binary.

The use of the __end__ variable is just more correct since it comes
from the linker script.