wic: add support to look in all layers and get .wks file

wic: add support to look in all layers and get .wks file

.wks file are looked in 'scripts/lib/image/canned-wks' directory on all
BBLAYERS variable returned by bitbake environment. If found, it will
be used.

The user could create your own .wks and keep it inside its layers. For
now the path must be <layer-dir>/scripts/lib/image/canned-wks.

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>