wic: Add BootimgEFIPlugin and BootimgPcbiosPlugin

wic: Add BootimgEFIPlugin and BootimgPcbiosPlugin

Implement the BootimgPcbiosPlugin and BootimgEFIPlugin SourcePlugin
classes.  The configure/prepare_partition() methods are implemented
using code derived from similar code in the Wic_PartData class.

These classes have the corresponding names 'bootimg-pcbios' and
'bootimg-efi', which are the names that should be used in the --source
parameters of the .wks partition commands.

(From OE-Core rev: 6e147488b40f730e07f1e0f232083ed75388daa0)

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