mekotronics-r58x-4g: u-boot: bump to v2026.04-rc2 with symlink-shared DT

- u-boot v2026.04-rcX has bumped dt-rebasing to v6.18, thus it knows about
  NPU nodes now and we can simply symlink to kernel DT, reducing duplication by a lot

Author: rpardini

config/boards/mekotronics-r58x-4g.csc

@@ -29,8 +29,8 @@ function post_family_config__mekor58x_4g_use_mainline_uboot() {
 	declare -g BOOTCONFIG="mekotronics-r58x-4g-rk3588_defconfig" # mainline
 	declare -g BOOTDELAY=1
 	declare -g BOOTSOURCE="https://github.com/u-boot/u-boot.git"
-	declare -g BOOTBRANCH="tag:v2026.01"
-	declare -g BOOTPATCHDIR="v2026.01"
+	declare -g BOOTBRANCH="tag:v2026.04-rc2"
+	declare -g BOOTPATCHDIR="v2026.04"
 	declare -g BOOTDIR="u-boot-${BOARD}"
 
 	declare -g UBOOT_TARGET_MAP="BL31=bl31.elf ROCKCHIP_TPL=${RKBIN_DIR}/${DDR_BLOB};;u-boot-rockchip.bin"