zynq: Make CONFIG_OF_EMBED default case

Use embedded DTB to let users use u-boot instead of u-boot-dtb.bin.
And fix SPL to use this target.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/configs/zynq_zybo_defconfig b/configs/zynq_zybo_defconfig
index 0c6117d..7779836 100644
--- a/configs/zynq_zybo_defconfig
+++ b/configs/zynq_zybo_defconfig
@@ -10,3 +10,4 @@
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_EMBED=y