utils: add enforce_gpt argument to get_efi_partition()

The Windows bootloader only supports a UEFI boot from a GPT
partition. Set enforce_gpt to True in every codepath related to
Windows. When enforce_gpt is set to True get_efi_partition()
raises an exception when an MBR partition scheme is detected.

src/utils/boot.py

@@ -57,7 +57,7 @@ def _boot_bios_windows(disk, part, mountpoint):
 def _boot_uefi_windows(disk, part, mountpoint):
     logging.info(f'Booting windows system')
     bootlabel = f'Part-{disk:02d}-{part:02d}'
-    esp, esp_disk, esp_part_number = get_efi_partition(disk)
+    esp, esp_disk, esp_part_number = get_efi_partition(disk, enforce_gpt=True)
     esp_mountpoint = esp.replace('dev', 'mnt')
     if not mount_mkdir(esp, esp_mountpoint):
         raise RuntimeError(f'Unable to mount detected EFI System Partition at {esp} into {esp_mountpoint}')
@@ -82,7 +82,7 @@ def _boot_uefi_windows(disk, part, mountpoint):
 def _boot_uefi_linux(disk, part, mountpoint):
     logging.info(f'Booting Linux system')
     bootlabel = f'Part-{disk:02d}-{part:02d}'
-    esp, esp_disk, esp_part_number = get_efi_partition(disk)
+    esp, esp_disk, esp_part_number = get_efi_partition(disk, enforce_gpt=False)
     esp_mountpoint = esp.replace('dev', 'mnt')
     if not mount_mkdir(esp, esp_mountpoint):
         raise RuntimeError(f'Unable to mount detected EFI System Partition at {esp} into {esp_mountpoint}')

src/utils/disk.py

@@ -41,9 +41,12 @@ def get_partition_device(disknum, partnum):
     raise ValueError(f'No such partition with disk index {disknum} and partition index {partnum}')
 
 
-def get_efi_partition(disknum):
+def get_efi_partition(disknum, enforce_gpt):
     """
-    Look for an EFI System Partition at the n-th disk. If disknum is invalid an exception is thrown
+    Look for an EFI System Partition at the n-th disk. If disknum is invalid an
+    exception is thrown.
+    If enforce_gpt is set to True the ESP will be ignored in a MBR partition
+    scheme.
 
     Returns tuple with:
         - Device name containing the ESP
@@ -57,7 +60,7 @@ def get_efi_partition(disknum):
     disk = get_disks()[disk_index]
     cxt = fdisk.Context(f'/dev/{disk}')
 
-    if cxt.label == fdisk.FDISK_DISKLABEL_DOS:
+    if enforce_gpt and cxt.label == fdisk.FDISK_DISKLABEL_DOS:
         raise RuntimeError(f'Disk has DOS partition scheme, cannot find ESP at /dev/{disk}')
 
     for pa in cxt.partitions: