utils: Expand relative paths in get_full_kernel_path()

When attempting to use '-k .' with boot-uml.py, the following stacktrace
is observed:

  $ .../boot-uml.py -k .
  $ linux ubd0=.../boot-utils/images/x86_64/rootfs.ext4
  Traceback (most recent call last):
    File ".../boot-utils/boot-uml.py", line 82, in <module>
      run_kernel(kernel, decomp_rootfs(), args.interactive)
    File ".../boot-utils/boot-uml.py", line 75, in run_kernel
      subprocess.run(uml_cmd, check=True)
    File "/usr/lib/python3.11/subprocess.py", line 548, in run
      with Popen(*popenargs, **kwargs) as process:
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "/usr/lib/python3.11/subprocess.py", line 1024, in __init__
      self._execute_child(args, executable, preexec_fn, close_fds,
    File "/usr/lib/python3.11/subprocess.py", line 1917, in _execute_child
      raise child_exception_type(errno_num, err_msg, err_filename)
  FileNotFoundError: [Errno 2] No such file or directory: PosixPath('linux')

This is because subprocess.run() tries to locate the linux binary in
PATH, not the current directory.

Fix this by always returning an absolute path from
get_full_kernel_path(), so that subprocess.run() uses the correct
binary.

Closes: #103
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: nathanchance

utils.py

@@ -90,7 +90,7 @@ def get_full_kernel_path(kernel_location, image, arch=None):
     if not kernel.exists():
         die(f"Kernel ('{kernel}') does not exist!")
 
-    return kernel
+    return kernel.resolve()
 
 
 def green(string):