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>

scripts/lib/image/canned-wks/directdisk.wks

@@ -3,7 +3,7 @@
 # can directly dd to boot media.
 
 
-part /boot --source bootimg --ondisk sda --fstype=msdos --label boot --active --align 1024
+part /boot --source bootimg-pcbios --ondisk sda --fstype=msdos --label boot --active --align 1024
 part / --source rootfs --ondisk sda --fstype=ext3 --label platform --align 1024
 
 bootloader  --timeout=0  --append="rootwait rootfstype=ext3 video=vesafb vga=0x318 console=tty0"

scripts/lib/image/canned-wks/mkefidisk.wks

@@ -2,7 +2,7 @@
 # long-description: Creates a partitioned EFI disk image that the user
 # can directly dd to boot media.
 
-part /boot --source bootimg --ondisk sda --fstype=efi --label msdos --active --align 1024
+part /boot --source bootimg-efi --ondisk sda --fstype=msdos --label msdos --active --align 1024
 
 part / --source rootfs --ondisk sda --fstype=ext3 --label platform --align 1024