convert-overrides.py: also convert comments without a leading whitespace

Currently lines like below are converted.
  e.g.
  # IMAGE_INSTALL_append = " A"

But lines without a leading whitespace are not converted.
  e.g.
  #IMAGE_INSTALL_append = " A"

We should be converting both.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>