u-boot: v2026.04: helios64: align ENV_OFFSET default with defconfig

Per CodeRabbit review on PR #9675:
  board/Kconfig ENV_OFFSET default 0x460000 conflicts with defconfig
  CONFIG_ENV_OFFSET=0x3F8000.

Authoritative source - upstream U-Boot itself:
  1. configs/helios64-rk3399_defconfig:7 sets CONFIG_ENV_OFFSET=0x3F8000
     - this is what actually ends up in the build; the defconfig
       overrides the Kconfig default.
  2. All 20+ rk3399 boards in upstream configs/*rk3399*_defconfig use
     0x3F8000 (evb, firefly, nanopi-m4/r4s, pinebook-pro, rock-4c,
     khadas-edge, leez, orangepi, ...).
  3. The 0x460000 in board/Kconfig is dead code: it is always shadowed
     by the defconfig, never applied in builds. It is a legacy artefact
     of the Kobol vendor patch series, not matching any standard SPI
     layout for rk3399.
  4. Physically: on Helios64's 16 MiB W25Q128JV, 0x3F8000 = 4 MiB - 32 KiB
     reserves the last 32 KiB slot inside the lower 4 MiB window for env
     - the common rk3399 convention.

Aligning the Kconfig default with the defconfig removes the
inconsistency and makes the fallback path land at the same address
the defconfig expects.

Author: iav

patch/u-boot/v2026.04/board_helios64/board/Kconfig

@@ -19,6 +19,6 @@ config ENV_SIZE
 	default 0x8000
 
 config ENV_OFFSET
-	default 0x460000 if ENV_IS_IN_SPI_FLASH
+	default 0x3F8000 if ENV_IS_IN_SPI_FLASH
 
 endif