stm32f429-discovery: add CONFIG_SYS_THUMB_BUILD

This target is ARMv7-M therefore can only build for Thumb,
but it did not #define CONFIG_SYS_THUMB_BUILD, so the U-Boot
code did not know it had to build for Thumb(2), not ARM.

This patch is binary-invariant: builds of stm32f429-discovery
with and without this patch were compared and found to differ
only by their U-Boot version strings.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>

include/configs/stm32f429-discovery.h

@@ -9,6 +9,7 @@
 #define __CONFIG_H
 
 #define CONFIG_STM32F4
+#define CONFIG_SYS_THUMB_BUILD
 #define CONFIG_STM32F4DISCOVERY
 
 #define CONFIG_OF_LIBFDT