rockchip: firefly: Use tiny-printf

Switch to using tiny-printf for the firefly SPL, this reduces the SPL by
around 1800 bytes bringing it back under the 32k limit for both gcc 4.9
and gcc 5.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>

configs/firefly-rk3288_defconfig

@@ -42,5 +42,6 @@ CONFIG_DEBUG_UART_CLOCK=24000000
 CONFIG_DEBUG_UART_SHIFT=2
 CONFIG_SYS_NS16550=y
 CONFIG_USE_PRIVATE_LIBGCC=y
+CONFIG_USE_TINY_PRINTF=y
 CONFIG_CMD_DHRYSTONE=y
 CONFIG_ERRNO_STR=y