feat(mtd): add platform function to allow using external buffer

The scratch buffer could be large. The new function allows
platform to defined its own external buffer or use the default
one.

Signed-off-by: Lionel Debieve <lionel.debieve@foss.st.com>
Change-Id: Ib7ab8ff19fa0a9cb06e364f058b91af58c3c471a
2 files changed