binman: Document how CROSS_COMPILE, CC, HOSTCC etc. are used in README

Explain that binman interprets these environment variables in the
"External tools" section to run target/host specific versions of the
tools, and add a new section on how to use CROSS_COMPILE to run the
tests on non-x86 machines.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

tools/binman/README

@@ -884,6 +884,12 @@ the 'tools' module's Run() method. The tools generally must exist on the PATH,
 but the --toolpath option can be used to specify additional search paths to
 use. This option can be specified multiple times to add more than one path.
 
+For some compile tools binman will use the versions specified by commonly-used
+environment variables like CC and HOSTCC for the C compiler, based on whether
+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.
+
 
 Code coverage
 -------------
@@ -918,6 +924,24 @@ directories so they can be examined later. Use -X or --test-preserve-dirs for
 this.
 
 
+Running tests on non-x86 architectures
+--------------------------------------
+
+Binman's tests have been written under the assumption that they'll be run on a
+x86-like host and there hasn't been an attempt to make them portable yet.
+However, it's possible to run the tests by cross-compiling to x86.
+
+To install an x86 cross-compiler on Debian-type distributions (e.g. Ubuntu):
+
+  $ sudo apt-get install gcc-x86-64-linux-gnu
+
+Then, you can run the tests under cross-compilation:
+
+  $ CROSS_COMPILE=x86_64-linux-gnu- binman test -T
+
+You can also use gcc-i686-linux-gnu similar to the above.
+
+
 Advanced Features / Technical docs
 ----------------------------------