boot/zephyr/prj.conf: remove SOC_FLASH_STM32F4, make it generic

Make it generic and let the SoC itself handle the FLASH driver support
as part of the Zephyr build system.

Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index 5c36b1d..1e6cda2 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -11,4 +11,3 @@
 CONFIG_HEAP_MEM_POOL_SIZE=16384
 
 CONFIG_FLASH=y
-CONFIG_SOC_FLASH_STM32F4=y