Update the documentation for SRCPV

The variable SRCPV is deprecated since 4.3. Instead of including SRCPV
in PV, including the sign "+" is enough for bitbake to add the source
control information to PKGV during the packaging phase.

Update the documentation for SRCPV and the places where it was used.
When instructions previously referred to SRCPV, replace by mentioning to
include "+" in the assignment.

In most examples, "+git" is added to PV as it is the most popular SCM.
Simply adding "+" is also possible, although it is better practice to
include the SCM name, so give that example.

Update the gcompat example with l3afpad as it didn't include "+git" in
its PV definition anymore.

(From yocto-docs rev: ef4d259842d9b1dd2d08ee38e00f932852f70543)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit ee16c96202e5027d1a8d7e89e11c25f127c78326)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/kernel-dev/common.rst

@@ -1562,16 +1562,9 @@ Here are some basic steps you can use to work with your own sources:
       changed.
 
    -  :term:`PV`: The default :term:`PV`
-      assignment is typically adequate. It combines the
-      :term:`LINUX_VERSION` with the Source Control Manager (SCM) revision
-      as derived from the :term:`SRCPV`
-      variable. The combined results are a string with the following
-      form::
-
-         3.19.11+git1+68a635bf8dfb64b02263c1ac80c948647cc76d5f_1+218bd8d2022b9852c60d32f0d770931e3cf343e2
-
-      While lengthy, the extra verbosity in :term:`PV` helps ensure you are
-      using the exact sources from which you intend to build.
+      assignment is typically adequate. It combines the value of
+      :term:`LINUX_VERSION` and the value ``+git`` which adds source control
+      information to :term:`PKGV` later during the packaging phase.
 
    -  :term:`COMPATIBLE_MACHINE`:
       A list of the machines supported by your new recipe. This variable