Increase CONFIG_SYS_BOOTM_LEN to 16M.

This solves the following uboot error:
Loading Kernel Image ... Image too large: increase CONFIG_SYS_BOOTM_LEN

Author: brocaar

layers/targets/meta-raspberrypi/recipes-bsp/u-boot/patches/rpi.patch

@@ -24,5 +24,5 @@ index f76c7d18..7d0d3562 100644
 +#define CONFIG_SILENT_CONSOLE
 +#define CONFIG_SYS_DEVICE_NULLDEV
 +#define CONFIG_BOARD_EARLY_INIT_F 1
-+
++#define CONFIG_SYS_BOOTM_LEN SZ_16M
  #endif