doc: update reference to README.board

Update reference in files detected by
scripts/documentation-file-ref-check

doc/README.<board>  => board/<board>/README

Files moved in board directory in
commit 702e6014f15b ("doc: cleanup - move board
READMEs into respective board directories")'

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
7 files changed