[raspberrypi-bootloader] Boot GPT with hybrid MBR fail

[rfried]

Update raspberrypi-bootloader from 20200212-1 to 20200714-1 break booting Raspberry Pi 2B, 3B, 3B+
using GPT with hybrid MBR partitioning.
Boot hangs with rainbow screen and harddisk led (green one) blinks in an intervall of repeating 7 short blinks.

Partition Layout:

$this->bbcode_second_pass_code('', '$ gdisk -l /dev/sdd
GPT fdisk (gdisk) version 1.0.4

Partition table scan:
MBR: hybrid
BSD: not present
APM: not present
GPT: present

Found valid GPT with hybrid MBR; using GPT.
Disk /dev/sdd: 62333952 sectors, 29.7 GiB
Model: USB3.0 CRW -SD
Sector size (logical/physical): 512/512 bytes
Disk identifier (GUID): BAEA9096-A7AE-48F3-9820-66887CA93EE0
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 62333918
Partitions will be aligned on 2048-sector boundaries
Total free space is 3621821 sectors (1.7 GiB)

Number Start (sector) End (sector) Size Code Name
1 8192 212991 100.0 MiB 0700 Microsoft basic data
2 212992 58720255 27.9 GiB 8E00 Linux LVM
')

$this->bbcode_second_pass_code('', '$ fdisk -t gpt -u -l /dev/sdd
Disk /dev/sdd: 29.74 GiB, 31914983424 bytes, 62333952 sectors
Disk model: USB3.0 CRW -SD
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: BAEA9096-A7AE-48F3-9820-66887CA93EE0

Device Start End Sectors Size Type
/dev/sdd1 8192 212991 204800 100M Microsoft basic data
/dev/sdd2 212992 58720255 58507264 27.9G Linux LVM


$ fdisk -t dos -u -l /dev/sdd
Disk /dev/sdd: 29.74 GiB, 31914983424 bytes, 62333952 sectors
Disk model: USB3.0 CRW -SD
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x00000000

Device Boot Start End Sectors Size Id Type
/dev/sdd1 * 8192 212991 204800 100M c W95 FAT32 (LBA)
/dev/sdd2 1 8191 8191 4M ee GPT
/dev/sdd4 212992 62333951 62120960 29.6G ee GPT

Partition table entries are not in disk order.
')

Reverting to MBR Partitioning by this (sdd and mmcblk0 is the same sd card) does allow booting again:
$this->bbcode_second_pass_code('', '$ sgdisk -z /dev/sdd

$ fdisk -u -l /dev/mmcblk0
Disk /dev/mmcblk0: 29.72 GiB, 31914983424 bytes, 62333952 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x00000000

Device Boot Start End Sectors Size Id Type
/dev/mmcblk0p1 * 8192 212991 204800 100M c W95 FAT32 (LBA)
/dev/mmcblk0p2 212992 58720255 58507264 27.9G 8e Linux LVM
')