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>
diff --git a/arch/arm/mach-stm32/Kconfig b/arch/arm/mach-stm32/Kconfig
index f70f5ec..947ce5f 100644
--- a/arch/arm/mach-stm32/Kconfig
+++ b/arch/arm/mach-stm32/Kconfig
@@ -18,7 +18,7 @@
 	select SPL_OF_CONTROL
 	select SPL_OF_LIBFDT
 	select SPL_OF_TRANSLATE
-	select SPL_OS_BOOT
+	imply SPL_OS_BOOT
 	select SPL_PINCTRL
 	select SPL_RAM
 	select SPL_SERIAL_SUPPORT