Add a new wic-specific bootloader subclass so we can add a --source
param to hang non-partition plugin off of.
By default, the bootloader gets the /boot partition source plugin, but
this can be overridden by the --source bootloader param if needed.
(From OE-Core rev: f90e4097c4e69d4f61c69923cb5d1ebb6b74d2ff)
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
