bootstage: Correct relocation algorithm

At present bootstage relocation assumes that it is possible to point back
to memory available before relocation, so it does not relocate the
strings. However this is not the case on some platforms, such as x86 which
uses the cache as RAM and loses access to this when the cache is enabled.

Move the relocation step to before U-Boot relocates, expand the allocated
region to include space for the strings and relocate the strings at the
same time as the bootstage records.

This ensures that bootstage data can remain accessible from TPL through
SPL to U-Boot before/after relocation.

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