ref-manual: variables: correct sdk installation default path

The SDKPATH variable seems mistakenly identified as the
default path where the SDK will be installed by the generated
installation script, unless option '-d' or a manual input
overrides this default.

The intended variable is SDKPATHINSTALL. SDKPATH indicates
where the SDK is being composed and built.

The definitions have been added/updated.

(From yocto-docs rev: ef86bec59f5bd81963e0013fce63a1960df53c57)

Signed-off-by: Johan Bezem <jbezem.extern@arri.de>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/overview-manual/concepts.rst

@@ -1375,7 +1375,7 @@ This next list, shows the variables associated with a standard SDK:
    Lists packages that make up the target part of the SDK (i.e. the part
    built for the target hardware).
 
--  :term:`SDKPATH`: Defines the
+-  :term:`SDKPATHINSTALL`: Defines the
    default SDK installation path offered by the installation script.
 
 -  :term:`SDK_HOST_MANIFEST`:

documentation/ref-manual/variables.rst

@@ -7612,6 +7612,10 @@ system and gives an overview of their function and contents.
          configuration will not take effect.
 
    :term:`SDKPATH`
+      Defines the path used to collect the SDK components and build the
+      installer.
+
+   :term:`SDKPATHINSTALL`
       Defines the path offered to the user for installation of the SDK that
       is generated by the OpenEmbedded build system. The path appears as
       the default location for installing the SDK when you run the SDK's
@@ -7621,7 +7625,7 @@ system and gives an overview of their function and contents.
    :term:`SDKTARGETSYSROOT`
       The full path to the sysroot used for cross-compilation within an SDK
       as it will be when installed into the default
-      :term:`SDKPATH`.
+      :term:`SDKPATHINSTALL`.
 
    :term:`SECTION`
       The section in which packages should be categorized. Package