meta-bytedance: Fix incorrect Yocto syntax

The `PREFERRED_PROVIDER` still uses `_` instead of `:`, fix it.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I34f27870ff35de8b14a8a546d42ab4a1fb5f30d1
diff --git a/meta-bytedance/meta-g220a/conf/machine/g220a.conf b/meta-bytedance/meta-g220a/conf/machine/g220a.conf
index f1a53d8..ba7a253 100644
--- a/meta-bytedance/meta-g220a/conf/machine/g220a.conf
+++ b/meta-bytedance/meta-g220a/conf/machine/g220a.conf
@@ -33,5 +33,5 @@
 PREFERRED_PROVIDER_virtual/obmc-system-mgmt = "packagegroup-bytedance-apps"
 
 PREFERRED_PROVIDER_virtual/bootloader = "u-boot-aspeed-sdk"
-PREFERRED_PROVIDER:u-boot = "u-boot-aspeed-sdk"
-PREFERRED_PROVIDER:u-boot-fw-utils = "u-boot-fw-utils-aspeed-sdk"
+PREFERRED_PROVIDER_u-boot = "u-boot-aspeed-sdk"
+PREFERRED_PROVIDER_u-boot-fw-utils = "u-boot-fw-utils-aspeed-sdk"