meta-aspeed: Correct the offset to load kernel

OpenBMC places kernel at offset 512 if the flash size is 32MB,
but it was defined as 1024 in the present defconfig.
This patch corrects the offset if the flash size is set to 32MB.

Signed-off-by: Howard Chiu <howard_chiu@aspeedtech.com>
Change-Id: I05749eda519cc1660921e111a36be74cb121fa68
diff --git a/meta-aspeed/conf/machine/evb-ast2500.conf b/meta-aspeed/conf/machine/evb-ast2500.conf
index 6946c52..4de380f 100644
--- a/meta-aspeed/conf/machine/evb-ast2500.conf
+++ b/meta-aspeed/conf/machine/evb-ast2500.conf
@@ -7,3 +7,5 @@
 require conf/machine/include/obmc-evb-common.inc
 
 SERIAL_CONSOLES = "115200;ttyS4"
+
+FLASH_SIZE = "32768"