tpl: Rename TPL_NEEDS_SEPARATE_STACK to TPL_HAVE_INIT_STACK

tpl: Rename TPL_NEEDS_SEPARATE_STACK to TPL_HAVE_INIT_STACK

The most common word for features that make a platform work is to use
'HAVE_xxx'. Rename this option to match.

Update the help to use the word 'phase' rather than 'stage', since
that is the current terminology. Also clarify that, absent this setting,
the stack pointer generally comes from the value used by U-Boot proper,
rather than SPL.

Move the option just above TPL_STACK which depends on it.

Signed-off-by: Simon Glass <sjg@chromium.org>