classes/image_ipk: Add support for multiple device tables. The original code

classes/image_ipk: Add support for multiple device tables. The original code
support a single value as IMAGE_DEVICE_TABLE which needed to be an absolute
value. If that is definied it'll be used as is and this change will have no
effect. If is is not definied it'll check for IMAGE_DEVICE_TABLES which can
be a list of device images tables which are searched for relative to the
BBPATH and it'll apply each one in turn. If neither of these are definied
it'll use the default of files/device_table-minimal.txt