docs: kbuild: add INSTALL_DTBS_PATH

The documentation for kbuild and makefiles is missing an explanation of
a variable important for some architectures.
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/kbuild.rst

@@ -243,6 +243,12 @@ The output directory is often set using "O=..." on the commandline.
 
 The value can be overridden in which case the default value is ignored.
 
+INSTALL_DTBS_PATH
+-----------------
+INSTALL_DTBS_PATH specifies where to install device tree blobs for
+relocations required by build roots.  This is not defined in the
+makefile but the argument can be passed to make if needed.
+
 KBUILD_ABS_SRCTREE
 --------------------------------------------------
 Kbuild uses a relative path to point to the tree when possible. For instance,

Documentation/kbuild/makefiles.rst

@@ -1623,6 +1623,13 @@ INSTALL_MOD_STRIP
   INSTALL_MOD_STRIP value will be used as the option(s) to the strip
   command.
 
+INSTALL_DTBS_PATH
+  This variable specifies a prefix for relocations required by build
+  roots. It defines a place for installing the device tree blobs. Like
+  INSTALL_MOD_PATH, it isn't defined in the Makefile, but can be passed
+  by the user if desired. Otherwise it defaults to the kernel install
+  path.
+
 Makefile language
 =================