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>
diff --git a/Makefile b/Makefile
index 57c3643..1885f36 100644
--- a/Makefile
+++ b/Makefile
@@ -1303,11 +1303,13 @@
 
 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) \