bootstage: Convert to use malloc()

At present bootstage uses the data section of the image to store its
information. There are a few problems with this:

- It does not work on all boards (e.g. those which run from flash before
relocation)
- Allocated strings still point back to the pre-relocation data after
relocation

Now that U-Boot has a pre-relocation malloc() we can use this instead,
with a pointer to the data in global_data. Update bootstage to do this and
set up an init routine to allocate the memory.

Now that we have a real init function, we can drop the fake 'reset' record
and add a normal one instead.

Note that part of the problem with allocated strings remains. They are
reallocated but this will only work where pre-relocation memory is
accessible after relocation.

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