revert 7f18485: improve uefi detection mechanism

Make is_uefi_supported() only check for /sys/firmware/efi as
get_efi_partition() will detect a missing ESP or an invalid
partition scheme. Stop using get_efi_partition() inside
is_uefi_supported() as the former is eventually called in every
UEFI related code.
UEFI supports both MBR and GPT as partition schemes and this is
a required change to handle the particular case of Windows not
being able to boot UEFI from a MBR partition scheme.

src/utils/boot.py

@@ -111,7 +111,7 @@ def boot_os_at(disk, part):
     if not mount_mkdir(device, mountpoint):
         raise RuntimeError(f'Cannot probe OS family. Unable to mount {device} at {esp_mountpoint}.')
 
-    is_uefi = is_uefi_supported(disk)
+    is_uefi = is_uefi_supported()
     if is_uefi:
         logging.info('UEFI support detected')
     else:

src/utils/uefi.py

@@ -11,7 +11,6 @@ import logging
 import os
 import shlex
 import subprocess
-from src.utils.disk import get_efi_partition
 
 import fdisk
 
@@ -47,17 +46,8 @@ def _check_efibootmgr_json():
     return supported
 
 
-def is_uefi_supported(disknum):
-    is_supported = os.path.exists("/sys/firmware/efi")
-
-    if is_supported:
-        try:
-            get_efi_partition(disknum)
-        except Exception as e:
-            logging.info(e)
-            is_supported = False
-
-    return is_supported
+def is_uefi_supported():
+    return os.path.exists("/sys/firmware/efi")
 
 
 def run_efibootmgr_json():