binman: Use a private directory for bintools

binman: Use a private directory for bintools

At present binman writes tools into the ~/bin directory. This is
convenient but some may be concerned about downloading unverified
binaries and running them. Place then in a special ~/.binman-tools
directory instead.

Mention this in the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>