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>