video/logo: do not generate unneeded logo C files

Currently, all the logo C files are generated irrespective of the
CONFIG options. Adding them to extra-y is wrong. What we need to do
here is to add them to 'targets' so that if_changed works properly.

Files listed in 'targets' are cleaned, so clean-files is unneeded.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>