configs: evb-ast2500: Move BOOTCOMMAND from header to defconfig

Move the BOOTCOMMAND definition from the board inclusion
header to the default configuration file.

Signed-off-by: Chia-Wei, Wang <chiawei_wang@aspeedtech.com>
diff --git a/configs/evb-ast2500_defconfig b/configs/evb-ast2500_defconfig
index 825fa4d..ce38b38 100644
--- a/configs/evb-ast2500_defconfig
+++ b/configs/evb-ast2500_defconfig
@@ -11,6 +11,8 @@
 CONFIG_DEFAULT_DEVICE_TREE="ast2500-evb"
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS4,115200n8 root=/dev/ram rw"
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="bootm 20080000 20300000"
 CONFIG_PRE_CONSOLE_BUFFER=y
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_HUSH_PARSER=y