bootimg.bbclass: Add parameter to specify HDDIMG volume ID
Users and children of the bootimg class may need to specify at build
time the volume ID of the HDDIMG vfat partition. An example of this
may be when at runtime the partition is uniquely identified by
the volume ID.
The HDDIMG_ID is expected to be provided externally. If the variable
is not set, mkdosfs will use the default volume-id.
