wic: add support to look in all layers and get plugins

wic: add support to look in all layers and get plugins

Plugins are looked in 'scripts/lib/mic/plugins/[type]/' directory on all
BBLAYERS variable returned by bitbake environment. If found, it will
be load at runtime.

The user could create your own plugin and keep it inside its layers. For
now the path must be <layer-dir>/scripts/lib/mic/plugins/[type]/. Where
'type' could be 'imager' or 'source'.

(From OE-Core rev: bb6f5d7de1c7ce2680874a74949903db0f5bb91a)

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>