devtool: make plugin_init optional

devtool: make plugin_init optional

So far all devtool and recipetool plugins were expected
to have plugin_init function. This function is empty in
most of plugins as they don't require initialisation.

Making plugin_init optional would allow not having empty
plugin_init in every plugin.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>