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.

(From OE-Core rev: b99640c89f067866b264f4ee4030fae4c2f338c0)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/recipetool/append.py

@@ -33,10 +33,6 @@ logger = logging.getLogger('recipetool')
 
 tinfoil = None
 
-def plugin_init(pluginlist):
-    # Don't need to do anything here right now, but plugins must have this function defined
-    pass
-
 def tinfoil_init(instance):
     global tinfoil
     tinfoil = instance