conmakehash uses getenv("abs_srctree") from the environment to strip
the absolute path from the generated sources.
However since commit e2bad142 ("kbuild: unexport abs_srctree and
abs_objtree") this environment variable no longer gets set.
Instead use basename() to indicate the used file in a comment of the
generated source file.

Fixes: 3bd85c6c ("tty: vt: conmakehash: Don't mention the full path of the input in output")
Cc: stable <stable@kernel.org>
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Link: https://lore.kernel.org/stable/20240725132056.9151-1-max.oss.09%40gmail.com
Link: https://lore.kernel.org/r/20240725132056.9151-1-max.oss.09@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>