bootstage: Allow counting memory without strings

bootstage: Allow counting memory without strings

The bootstage array includes pointers to strings but not the strings
themselves. The strings are added when stashing, but including them in
the size calculation gives an inflated view of the amount of space used
by the array.

Update this function so it can return the amount of memory used by the
bootstage structures themselves, without the strings which they point
to.

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