wic: Hook up BootimgEFIPlugin and BootimgPcbiosPlugin plugins

wic: Hook up BootimgEFIPlugin and BootimgPcbiosPlugin plugins

Remove all the Wic_PartData and DirectImageCreator code now
implemented by the BootimgEFIPlugin and BootimgPcbiosPlugin plugins,
as well as all the special-cased boot_type code, significantly
cleaning up the code.

Replace the calling code with general-purpose plugin invocations, in
essence calling the appropriate implementations at run-time based on
the --source value in effect.

Change the directdisk.wks and mkefidisk.wks scripts to make use of the
new plugins.

(From OE-Core rev: 43558610a5793888ff2b18bd3a27c7ab558e5ad0)

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>