binman: Allow providing tools and blob directories

At present it is necessary to symlink files containing external blobs into
the U-Boot tree in order for binman to find them. This is not very
convenient.

Add two new environment/Makefile variables to help with this. Add
documentation as well, fixing a related nit.

Signed-off-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -1303,11 +1303,13 @@ default_dt := $(if $(DEVICE_TREE),$(DEVICE_TREE),$(CONFIG_DEFAULT_DEVICE_TREE))
 
 quiet_cmd_binman = BINMAN  $@
 cmd_binman = $(srctree)/tools/binman/binman $(if $(BINMAN_DEBUG),-D) \
+		$(foreach f,$(BINMAN_TOOLPATHS),--toolpath $(f)) \
                 --toolpath $(objtree)/tools \
 		$(if $(BINMAN_VERBOSE),-v$(BINMAN_VERBOSE)) \
 		build -u -d u-boot.dtb -O . -m --allow-missing \
 		-I . -I $(srctree) -I $(srctree)/board/$(BOARDDIR) \
 		-I arch/$(ARCH)/dts -a of-list=$(CONFIG_OF_LIST) \
+		$(foreach f,$(BINMAN_INDIRS),-I $(f)) \
 		-a atf-bl31-path=${BL31} \
 		-a opensbi-path=${OPENSBI} \
 		-a default-dt=$(default_dt) \

tools/binman/binman.rst

@@ -942,7 +942,7 @@ Replacing files in an image
 ---------------------------
 
 You can replace files in an existing firmware image created by binman, provided
-that there is an 'fdtmap' entry in the image. For example:
+that there is an 'fdtmap' entry in the image. For example::
 
     $ binman replace -i image.bin section/cbfs/u-boot
 
@@ -1081,6 +1081,35 @@ the tool's output will be used for the target or for the host machine. If those
 aren't given, it will also try to derive target-specific versions from the
 CROSS_COMPILE environment variable during a cross-compilation.
 
+If the tool is not available in the path you can use BINMAN_TOOLPATHS to specify
+a space-separated list of paths to search, e.g.::
+
+   BINMAN_TOOLPATHS="/tools/g12a /tools/tegra" binman ...
+
+
+External blobs
+--------------
+
+Binary blobs, even if the source code is available, complicate building
+firmware. The instructions can involve multiple steps and the binaries may be
+hard to build or obtain. Binman at least provides a unified description of how
+to build the final image, no matter what steps are needed to get there.
+
+Binman also provides a `blob-ext` entry type that pulls in a binary blob from an
+external file. If the file is missing, binman can optionally complete the build
+and just report a warning. Use the `-M/--allow-missing` option to enble this.
+This is useful in CI systems which want to check that everything is correct but
+don't have access to the blobs.
+
+If the blobs are in a different directory, you can specify this with the `-I`
+option.
+
+For U-Boot, you can use set the BINMAN_INDIRS environment variable to provide a
+space-separated list of directories to search for binary blobs::
+
+   BINMAN_INDIRS="odroid-c4/fip/g12a \
+       odroid-c4/build/board/hardkernel/odroidc4/firmware \
+       odroid-c4/build/scp_task" binman ...
 
 Code coverage
 -------------