bootimage.bbclass: Move fat image creation into a function
In order to call the fat image creation multiple times it needs to be
in its own function. A future commit will make use of the new
function to additionally create EFI image files for use with an ISO.
[YOCTO #4100]
[YOCTO #1913]
Signed-off-by: Jason Wessel <jason.wessel@windriver.com> Signed-off-by: Saul Wold <sgw@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
