poppy: Enable optimized SHA256 implementation
BRANCH=none
TEST=Boot soraka, hash done time goes down from ~1.30s to ~1.16s,
with a ~750 bytes code size increase.
#define CONFIG_SPI_FLASH_W25X40
#define CONFIG_UART_HOST 0
#define CONFIG_VBOOT_HASH
+#define CONFIG_SHA256_UNROLLED
#define CONFIG_VSTORE
#define CONFIG_VSTORE_SLOT_COUNT 1
#define CONFIG_WATCHDOG_HELP
