nanopct6(-lts): if using mainline u-boot, let u-boot determine fdtfile

config/boards/nanopct6.conf

@@ -51,6 +51,13 @@ function post_family_config_branch_edge__nanopct6_use_mainline_uboot() {
 	}
 }
 
+# U-boot 2025.04+ can detect and set fdtfile automatically across T6 and T6-LTS boards.
+# So if using mainline u-boot, unset BOOT_FDT_FILE to let u-boot handle it.
+# That way, both variants can boot from the same image; lets keep the -lts board file for vendor kernel/u-boot.
+function post_family_config_branch_edge__t6_and_t6_lts_auto_dtb_name_via_uboot_detection() {
+	unset BOOT_FDT_FILE
+}
+
 function pre_config_uboot_target__nanoptc6_patch_uboot_dtsi_for_ums() {
 	[[ "${BRANCH}" != "edge" ]] && return 0