meta-facebook: yosemite4: u-boot-nuvoton: support to use `preboot`

Add configurations to use `preboot` feature for testing ddr.

Tested:
root@bmc:~# fw_setenv preboot 'mtest 0x10000000 0x30000000 0x123 10'
After that, reboot BMC then `mtest` will run as expected before kernel.

Testing 10000000 ... 30000000:
Pattern FFFFFFFFFFFFFEDC  Writing...  Reading...Iteration:     16
Tested 16 iteration(s) with 0 errors.

Signed-off-by: Tim Lee <timlee660101@gmail.com>
Change-Id: Iaaec934db1914ddc164d2ad453e2d955f8dedd13
diff --git a/meta-facebook/meta-yosemite4/meta-yosemite4n/recipes-bsp/u-boot/u-boot-nuvoton/yosemite4.cfg b/meta-facebook/meta-yosemite4/meta-yosemite4n/recipes-bsp/u-boot/u-boot-nuvoton/yosemite4.cfg
index 096f5bd..92f45a5 100644
--- a/meta-facebook/meta-yosemite4/meta-yosemite4n/recipes-bsp/u-boot/u-boot-nuvoton/yosemite4.cfg
+++ b/meta-facebook/meta-yosemite4/meta-yosemite4n/recipes-bsp/u-boot/u-boot-nuvoton/yosemite4.cfg
@@ -1,3 +1,6 @@
 CONFIG_SYS_SKIP_UART_INIT=y
 CONFIG_USE_BOOTCOMMAND=y
 CONFIG_BOOTCOMMAND="setenv console ttyS4,57600n8; run common_bootargs; bootm 80300000"
+CONFIG_USE_PREBOOT=y
+CONFIG_ENV_OFFSET=0x2C0000
+CONFIG_ENV_ADDR=0x802C0000