board: stm32: Fix stm32f746-disco boot

Since 'commit af2f4426 ("fdc spl: use different BOARD_INIT MACRO for spl and tpl")'
board stm32f746-disco can't boot.

The macro CONFIG_IS_ENABLED() can't evaluate the value of
CONFIG_SPL_BOARD_INIT as it was defined in include/configs/stm32f746-disco.h
without attributed value.

Moving CONFIG_SPL_BOARD_INIT from stm32f746-disco.h to mach-stm32/Kconfig
fixes this issue.

Fixes: af2f4426 ("fdc spl: use different BOARD_INIT MACRO for spl and tpl")
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>

arch/arm/mach-stm32/Kconfig

@@ -29,6 +29,7 @@ config STM32F7
 	select STM32_SERIAL
 	select SUPPORT_SPL
 	select SPL
+	select SPL_BOARD_INIT
 	select SPL_CLK
 	select SPL_DM
 	select SPL_DM_SEQ_ALIAS

include/configs/stm32f746-disco.h

@@ -72,7 +72,6 @@
 #ifdef CONFIG_SUPPORT_SPL
 #define CONFIG_SPL_STACK		CONFIG_SYS_INIT_SP_ADDR
 #define CONFIG_SPL_FRAMEWORK
-#define CONFIG_SPL_BOARD_INIT
 #define CONFIG_SPL_TEXT_BASE		CONFIG_SYS_FLASH_BASE
 #define CONFIG_SYS_MONITOR_LEN		(512 * 1024)
 #define CONFIG_SYS_SPL_LEN		0x00008000