zephyr: move STM32 target config to soc_family_stm32.h

This cleanup commit requires a bit more target-specific knowledge
that's a bit harder to read than board-specific headers, but it's
still an improvement over what we have now.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
diff --git a/boot/zephyr/include/target.h b/boot/zephyr/include/target.h
index d37a523..101c572 100644
--- a/boot/zephyr/include/target.h
+++ b/boot/zephyr/include/target.h
@@ -14,6 +14,8 @@
 /* SoC family configuration. */
 #if defined(CONFIG_SOC_FAMILY_NRF5)
 #include "soc_family_nrf5.h"
+#elif defined(CONFIG_SOC_FAMILY_STM32)
+#include "soc_family_stm32.h"
 #endif
 
 /*