spl: stm32: make falcon mode activation configurable

With this change, it will be possible to de-select falcon mode & spl
will only boot U-Boot.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
Suggested-by: Bo Shen <voice.shen@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>