boards: st: add support for boards compatible with swap using scratch

SomeSTM32 series, like F2, F4, and F7, do not have homogeneous flash
sector sizes; they include very large sectors such as 128 KB
and even 256 KB.

The scratch partition needs to be large enough to hold the largest
sector size ( 256k for F7 and 128k for F2).

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>