The original code in intel-iot-refkit allows to create more than one
UEFI combo app and uses that to create one for removable media and one
for fixed media (after installation), with different boot=PARTUUID=xxx
parameters. This way, an installed image never ended up booting from
the install media.

uefi-comboapp.bbclass now supports the same feature, with
create_uefiapp() as the API function that can be used to create
additional UEFI apps and create_uefiapps as the method where the call
can be added.

In addition, several shortcomings are getting addressed:
- A UEFI combo app must be stored under a name that is specific
  to the image for which it gets created, otherwise different
  image recipes end up overwriting (or using) files from other
  images.
- Signing must be done after creating the apps and before deploying
  them, otherwise the unsigned apps get copied to the image when
  using do_uefiapp_deploy.
- The common code for deployment is now in uefiapp_deploy_at.
- $dest is used instead of ${DEST} because the latter might get
  expanded by bitbake.
- Because do_uefiapp always had to run anew to produce the
  clean, unsigned input for do_uefiapp_sign, having two different
  tasks just added unnecessary complexity. Now all code is in
  do_uefiapp.
- Old files matching the output pattern get removed explicitly,
  because they might not get overwritten when the optional
  app suffix changes between builds, or when the task fails
  in the middle.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>