binman: Allow resolving host-specific tools from env vars

This patch lets tools.Run() use host-specific versions with the
for_host keyword argument, based on the host-specific environment
variables (HOSTCC, HOSTOBJCOPY, HOSTSTRIP, etc.).

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