u-boot-hw23: Set empty initial environment

The default behavior is to try to build u-boot-initial-env
Which is not implemented in our u-boot repo.

Part of yocto update to dunfell release.

BugzID: 69636

recipes-bsp/u-boot/u-boot-imx8-nmhw23_git.bb

@@ -21,6 +21,7 @@ S = "${WORKDIR}/git"
 LOCALVERSION ?= "-${SRCBRANCH}"
 
 BOOT_TOOLS = "imx-boot-tools"
+UBOOT_INITIAL_ENV = ""
 
 do_configure_prepend() {
     if [ ! -d ${S}/board/netmodule/common ]; then