ref-manual: variables: add SIGGEN_LOCKEDSIGS* variables

Variables SIGGEN_LOCKEDSIGS, SIGGEN_LOCKEDSIGS_TASKSIG_CHECK and
SIGGEN_LOCKEDSIGS_TYPES are used to lock specific tasks to specific
signatures. They are used by bitbake -S <lockedsigs> and bblock, so add
documentation for them.

(From yocto-docs rev: 32e3995bed2836f549866ec3b8ad254bdda37dbf)

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Reviewed-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -7880,6 +7880,50 @@ system and gives an overview of their function and contents.
          might break at runtime if the interface of the recipe was changed
          after the other had been built.
 
+   :term:`SIGGEN_LOCKEDSIGS`
+     The list of locked tasks, with the form::
+
+       SIGGEN_LOCKEDSIGS += "<package>:<task>:<signature>"
+
+     If ``<signature>`` exists for the specified ``<task>`` and ``<package>``
+     in the sstate cache, BitBake will use the cached output instead of
+     rebuilding the ``<task>``. If it does not exist, BitBake will build the
+     ``<task>`` and the sstate cache will be used next time.
+
+     Example::
+
+       SIGGEN_LOCKEDSIGS += "bc:do_compile:09772aa4532512baf96d433484f27234d4b7c11dd9cda0d6f56fa1b7ce6f25f0"
+
+     You can obtain the signature of all the tasks for the recipe ``bc`` using::
+
+       bitbake -S none bc
+
+     Then you can look at files in ``build/tmp/stamps/<arch>/bc`` and look for
+     files like: ``<PV>.do_compile.sigdata.09772aa4532512baf96d433484f27234d4b7c11dd9cda0d6f56fa1b7ce6f25f0``.
+
+   :term:`SIGGEN_LOCKEDSIGS_TASKSIG_CHECK`
+     Specifies the debug level of task signature check. 3 levels are supported:
+
+     * ``info``: displays a "Note" message to remind the user that a task is locked
+       and the current signature matches the locked one.
+     * ``warn``: displays a "Warning" message if a task is locked and the current
+       signature does not match the locked one.
+     * ``error``: same as warn but displays an "Error" message and aborts.
+
+   :term:`SIGGEN_LOCKEDSIGS_TYPES`
+     Allowed overrides for :term:`SIGGEN_LOCKEDSIGS`. This is mainly used
+     for architecture specific locks. A common value for
+     :term:`SIGGEN_LOCKEDSIGS_TYPES` is ``${PACKAGE_ARCHS}``::
+
+       SIGGEN_LOCKEDSIGS_TYPES += "${PACKAGE_ARCHS}"
+
+       SIGGEN_LOCKEDSIGS_core2-64 += "bc:do_compile:09772aa4532512baf96d433484f27234d4b7c11dd9cda0d6f56fa1b7ce6f25f0"
+       SIGGEN_LOCKEDSIGS_cortexa57 += "bc:do_compile:12178eb6d55ef602a8fe638e49862fd247e07b228f0f08967697b655bfe4bb61"
+
+     Here, the ``do_compile`` task from ``bc`` will be locked only for
+     ``core2-64`` and ``cortexa57`` but not for other architectures such as
+     ``mips32r2``.
+
    :term:`SITEINFO_BITS`
       Specifies the number of bits for the target system CPU. The value
       should be either "32" or "64".