webos_enhanced_submissions: Make it easier to use AUTOREV

:Release Notes:
Fix issues we've discovered when using AUTOREV and SRCREVs without
matching tags.

:Detailed Notes:
Allow empty SRCREV in WEBOS_VERSION when WEBOS_SUBMISSION is 0.
Clear WEBOS_GIT_TAG (tag param in SRC_URI) when we're using SRCREV.

There are 3 typical cases when you would want to override the
WEBOS_VERSION setting in the recipe:
1) To use some specific SHA-1 which doesn't have matching submissions
   tag.

   SRCREV_pn-luna-service2 = "be814b74577110b7d80a0308ffc0869196df410e"
   # set the version, disable tag check
   WEBOS_VERSION_pn-luna-service2 = "3.7.0-0"
   # append suffix with revision so it's clear what was built
   PV_append_pn-luna-service2 = "${WEBOS_PV_SUFFIX}"

   This is for webos-local.conf, if you do it from .bbappend you can
   drop overrides ("_pn-luna-service2")

2) To use latest revision in selected branch.

   Same as 2) above, only difference is SRCREV variable:
   SRCREV_pn-luna-service2 = "${AUTOREV}"

   If you want to use latest revision in different branch add branch param
   to WEBOS_VERSION, e.g.
   WEBOS_VERSION_pn-luna-service2 = "3.7.0-0;branch=something"

3) To use some review on gerrit

   # set the version, disable tag check
   WEBOS_VERSION_pn-luna-service2 = "3.7.0-0"
   # disable branch check (as the review probably isn't included in
   # any branch - otherwise you would just set SRCREV)
   WEBOS_GIT_BRANCH_pn-luna-service2 = ";nobranch=1"
   # review reference
   WEBOS_GIT_PARAM_TAG_pn-luna-service2 = "refs/changes/09/5009/7"
   # enable tag param in SRC_URI
   WEBOS_GIT_TAG_pn-luna-service2 = ";tag=${WEBOS_GIT_PARAM_TAG}"
   # append suffix with revision so it's clear what was built;
   # you can use something else (like "+review5009"), the SHA-1
   # shown in package version (from SRCPV which is included in
   # WEBOS_PV_SUFFIX) is available only after you use:
   # git fetch <repo> refs/changes/09/5009/7
   PV_append_pn-luna-service2 = "${WEBOS_PV_SUFFIX}"

In all 3 cases you will see QA error about using submission 0, if
you want to change this error to warning, then add:
   WEBOS_ERROR_QA_AUTOREV_pn-luna-service2 = ""
but make sure that you'll never push such change to official build.

:Testing Performed:
Only build tested.

:QA Notes:
No change to image.

:Build Notes:
Invalidates all sstate signatures.

:Issues Addressed:
[ES-563] Make it easier to use AUTOREV (or any given SHA-1 or gerrit
         ref) from WEBOS_VERSION and document it

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I0636434f01580b968e473a70c006abd10a4645e3
Reviewed-on: https://g2g.palm.com/5023
Reviewed-by: DCO Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>