webos.inc: Use WEBOS_DISTRO_BRANCHPOINT_MICRO_VERSION in image filenames

:Release Notes:
Invent WEBOS_DISTRO_BRANCHPOINT_MICRO_VERSION, which is the official
build number, prefixed with the build codename, at which the branch for
a micro version was made. When defined, it appears instead of
WEBOS_DISTRO_BUILD_CODENAME in image filenames.

Also, allow the WEBOS_DISTRO_BUILD_CODENAME portion of IMAGE_NAME to be
overridden by setting WEBOS_IMAGE_BUILD_CODENAME.

:Detailed Notes:
The hyphen in WEBOS_DISTRO_BRANCHPOINT_MICRO_VERSION is replaced by a
period when forming the image filenames.

:Testing Performed:
Confirmed using:
  $ bitbake -e starfish-image | grep '^IMAGE_NAME='
that WEBOS_DISTRO_BRANCHPOINT_MICRO_VERSION was present instead of
WEBOS_DISTRO_BUILD_CODENAME when it was defined.

:QA Notes:

:Issues Addressed:
[GF-61406] Add support for branching for micro version releases
[GF-61414] CCC: Add support for branching for micro version releases

Open-webOS-DCO-1.0-Signed-off-by: Herb Kuta <herb.kuta@lge.com>
Open-webOS-DCO-1.0-Signed-off-by: Susan Montooth <susan.montooth@lge.com>

Change-Id: Iebf3e5c68999967a34741b740b97a9c44b812a0a
Originally-reviewed-on: https://g2g.palm.com/4806
Cherry-picked-from-commit: 67978debf46bfd93dc830433310f03941142db26
Cherry-picked-from-branch: @acadia