utils: add get_filesystem_type function

Retrieve filesystem type from a partition using get_filesystem_type.
Encapsulates a subprocess call to blkid.

src/utils/fs.py

@@ -165,3 +165,15 @@ def mkfs_fat32(partdev, label=None):
         subprocess.run(cmd,
                        stdout=logfile,
                        stderr=STDOUT)
+
+
+def get_filesystem_type(partdev):
+    """
+    Get filesystem type from a partition device path.
+    Raises RuntimeError when blkid exits with non-zero return code.
+    """
+    cmd = shlex.split(f'blkid -o value -s TYPE {partdev}')
+    proc = subprocess.run(cmd, stdout=PIPE, encoding='utf-8')
+    if proc.returncode != 0:
+        raise RuntimeError(f'Error getting filesystem from {partdev}')
+    return proc.stdout.strip()