core: document the stack location, use STACK_TOP throughout

core: document the stack location, use STACK_TOP throughout

The global absolute symbol STACK_TOP defined in layout.inc can be used
by the linker script as well as by anything else that needs to know
where the stack is.  Also document why we do it differently for
PXELINUX.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>