[WIP] Switch to a Hybrid MBR/GPT partition scheme.

- rootfs based on PARTUUID

buildroot-external/board/raspberrypi/cmdline.txt

@@ -1 +1 @@
-dwc_otg.lpm_enable=0 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes console=ttyAMA0,115200 consoleblank=0 loglevel=0 vt.global_cursor_default=0 logo.nologo systemd.show_status=0 rootwait quiet splash
+dwc_otg.lpm_enable=0 root=PARTUUID=c0932a41-44cf-463b-8152-d43188553ed4 rootfstype=ext4 elevator=deadline fsck.repair=yes console=ttyAMA0,115200 consoleblank=0 loglevel=0 vt.global_cursor_default=0 logo.nologo systemd.show_status=0 rootwait quiet splash

buildroot-external/board/raspberrypi/genimage-raspberrypi4-64.cfg

@@ -15,16 +15,19 @@ image boot.vfat {
 
 image sdcard.img {
   hdimage {
+    gpt = "true"
   }
 
-  partition boot {
+  partition boot1 {
     partition-type = 0xC
+    partition-type-uuid = c12a7328-f81f-11d2-ba4b-00a0c93ec93b
     bootable = "true"
     image = "boot.vfat"
   }
 
-  partition rootfs {
-    partition-type = 0x83
+  partition rootfs1 {
+    partition-uuid = c0932a41-44cf-463b-8152-d43188553ed4
+    partition-type-uuid = b921b045-1df0-41c3-af44-4c6f280d3fae
     image = "rootfs.ext4"
   }
 }

buildroot-external/configs/rpi4_64-gui_defconfig

@@ -472,7 +472,9 @@ BR2_PACKAGE_VIM=y
 # BR2_PACKAGE_VIM_RUNTIME is not set
 BR2_TARGET_ROOTFS_EXT2=y
 BR2_TARGET_ROOTFS_EXT2_4=y
+BR2_TARGET_ROOTFS_EXT2_LABEL="rootfs1"
 BR2_TARGET_ROOTFS_EXT2_SIZE="2527M"
+BR2_TARGET_ROOTFS_EXT2_MKFS_OPTIONS="-O 64bit -U c0932a41-44cf-463b-8152-d43188553ed4"
 # BR2_TARGET_ROOTFS_TAR is not set
 BR2_PACKAGE_HOST_CMAKE=y
 BR2_PACKAGE_HOST_DOSFSTOOLS=y

buildroot-external/rootfs-overlay/etc/fstab

@@ -1,5 +1,4 @@
 # <file system>	<mount pt>	<type>	<options>	<dump>	<pass>
-/dev/root	/		ext2	rw,noauto,x-systemd.growfs	0	1
 proc		/proc		proc	defaults	0	0
 devpts		/dev/pts	devpts	defaults,gid=5,mode=620,ptmxmode=0666	0	0
 tmpfs		/dev/shm	tmpfs	mode=0777	0	0