configs/am574x_idk_defconfig: bump linux to 6.18.16 and u-boot to 2026.04

This commit:
  - updates the Kernel from LTS 6.6.14 to 6.18.16
  - updates U-Boot from 2024.01 to 2026.04
  - switches to Bootlin external glibc stable toolchain
  - enable BR2_DOWNLOAD_FORCE_CHECK_HASHES=y, add custom hashes stored
    in BR2_GLOBAL_PATCH_DIR="board/ti/am574x-idk/patches" and removes
    the exception entry in .checkpackageignore.
  - enable BR2_LINUX_KERNEL_DTB_KEEP_DIRNAME=y to install the board
    devicetree in the boot (vfat) partition keeping the directory
    prefixes (ti/omap/am574x-idk.dtb). u-boot nowadays is looking for
    boards devicetree using such directory prefixes [1].
    Update genimange.cfg file accordingly.
  - While at it, add a label to the boot (vfat) partition to not use a
    generated label.
  - set BR2_TARGET_ROOTFS_EXT2_SIZE="256M" since the rootfs no longer
    fit in the default size (60M).
  - Remove the rootfs partition predefined 1G size to avoid a parted
    warning about "unallocated space within the partition."
  - Update extlinux.conf kernel command line to use "console=ttyS2"
    instead of "ttyS0". Seems a typo introduced with the defconfig
    (AM57xx always used ttyS2 as kernel console).

[1] 649f4a7d3c

Signed-off-by: Romain Naour <romain.naour@smile.fr>
[Julien: add BR2_TARGET_UBOOT_NEEDS_GNUTLS=y]
Signed-off-by: Julien Olivain <ju.o@free.fr>

board/ti/am574x-idk/extlinux.conf

@@ -1,4 +1,4 @@
 label am5749-idk-buildroot
   kernel /zImage
   fdtdir /
-  append console=ttyS0,115200n8 root=/dev/mmcblk0p2 rw rootfstype=ext4 rootwait
+  append console=ttyS2,115200n8 root=/dev/mmcblk0p2 rw rootfstype=ext4 rootwait

board/ti/am574x-idk/genimage.cfg

@@ -4,9 +4,11 @@ image boot.vfat {
 			"MLO",
 			"u-boot.img",
 			"zImage",
-			"am574x-idk.dtb",
+			"ti",
 			"extlinux"
 		}
+
+		label = "boot"
 	}
 
 	size = 16M
@@ -25,6 +27,5 @@ image sdcard.img {
 	partition rootfs {
 		partition-type = 0x83
 		image = "rootfs.ext4"
-		size = 1G
 	}
 }

board/ti/am574x-idk/patches/linux/linux.hash

@@ -0,0 +1,2 @@
+# From https://www.kernel.org/pub/linux/kernel/v6.x/sha256sums.asc
+sha256  f4855f382c1b735c84072bdef36db5bcd5dc7b0c37e42f5104317149a0a486ef  linux-6.18.18.tar.xz

board/ti/am574x-idk/patches/uboot/uboot.hash

@@ -0,0 +1,2 @@
+# Locally calculated
+sha256  ac7c04b8b7004923b00a4e5d6699c5df4d21233bac9fda690d8cfbc209fff2fd  u-boot-2026.04.tar.bz2